I thought it would be interesting to do a search for the use of the exact expression "Buddha Nature" by Theravadins. Here's what comes up from searching the site what-buddha-taught.net. I tried to check each one to see if they are "Theravada" - seems so to me. With possible exception of the poet "Matrceta" - but his poems look like they're referenced in Theravada circles?
Enjoy. Luang Por Liem Áhitadhammohttp://www.what-buddha-taught.net/Books ... reness.pdf
Opening our eyes will enable us to see nature within
ourselves, and our own inner Buddha-nature:
awakening. The hardships of poverty, and the allure
of wealth: even the power that these things can hold
over us can not separate us from our own essential
nature.Ven Dr K Sri Dhammanandahttp://www.what-buddha-taught.net/Books ... ankind.pdf
"Buddhists believe that in every living being lies hidden the Buddha-seed or Buddha-
nature waiting to ripen. That means, in every person there exists the potential to
become pure like the Buddha."
"What meaning has the Enlightenment of the Buddha for us? Firstly, the Buddha made us
realise our true potential for Enlightenment, that is, our Buddha nature"Guy Armstrong
in preface to Small Boat, Great Mountain by Ajahn Amarohttp://www.what-buddha-taught.net/Books ... untain.pdf
"So perhaps it should not be too surprising that in the modern
Thai forest tradition we find an understanding of Dharma with
strong parallels to the central tenets of Mahayana and Tibetan
Buddhism. The Mahayana doctrine of Buddha-nature, for
instance, tells us that our very essence is an unborn and undying
awareness. In a later expression of the teachings through the
Dzogchen school, specific meditation techniques have been
developed to allow practitioners to recognize and abide in this
nature. Ajahn Amaro (whose name means “deathless”)
commented that this specific teaching is the national anthem of
the Thai forest tradition."
"May their message lead all those who read them
directly to their own Buddha-nature and to the vast freedom of
the Natural Great Perfection."Ajahn Chahhttp://www.what-buddha-taught.net/Books ... reedom.htm
translated by Venerable Ajahn Puriso
"So whether standing, walking, sitting or lying we should have sati to watch over and look after the mind. When we see external things it's like seeing internals. When we see internals it's the same as seeing externals. If we understand this then we can hear the teaching of the Buddha. If we understand this, then we can say that Buddha-nature, the 'One who knows', has been established. It knows the external. It knows the internal. It understands all things which arise. Understanding like this, then sitting at the foot of a tree we hear the Buddha's teaching. Standing, walking, sitting or lying, we hear the Buddha's teaching. Seeing, hearing, smelling, tasting, touching and thinking, we hear the Buddha's teaching. The Buddha is just this 'One who knows' within this very mind. It knows the Dhamma, it investigates the Dhamma. It's not that the Buddha-nature, the 'one who knows', arises. The mind becomes illumined."
"When Añña Kondañña, the first disciple, heard the Buddha's teaching for the first time, the realization he had was nothing very complicated. He simply saw that whatever thing is born, that thing must change and grow old as a natural condition and eventually it must die. Añña Kondañña had never thought of this before, or if he had it wasn't thoroughly clear, so he hadn't yet let go, he still clung to the khandhas. As he sat mindfully listening to the Buddha's discourse, Buddha-nature arose in him. He received a sort of Dhamma "transmission," which was the knowledge that all conditioned things are impermanent. Any thing which is born must have aging and death as a natural result."
"The eight worldly dhammas are right here in this very mind, with the 'One who knows' but this 'One who knows' has obstructions, so it knows wrongly and thus becomes the world. It's just this one 'One who knows', no other! The Buddha-nature has not yet arisen in this mind, it has not yet extracted itself from the world. The mind like this is the world."
"There's a story in the scriptures about the Buddha, before he was enlightened. At that time, having received a plate of rice, he floated that plate on a stream of water, determining in his mind, "If I am to be enlightened, may this plate float against the current of the water." The plate floated upstream! That plate was the Buddha's right view, or the Buddha-nature that he became awakened to. It didn't follow the desires of ordinary beings. It floated against the flow of his mind, it was contrary in every way."http://www.what-buddha-taught.net/Books ... Part_2.htm
"Many people contend that since the mind is inherently pure, since we all have Buddha nature, it's not necessary to practice. But this is like taking something clean, like this tray, for example, and then I come and drop some dung on it. Will you say that this tray is originally clean, and so you don't have to do anything to clean it now?"Matrcetahttp://www.what-buddha-taught.net/acces ... el360.html
114 With mind detached, you quietly work
for the welfare of the world.
How awesome is the Buddha-nature of the Buddha! 
30. Buddhadharmata. See Encyclopedia of Buddhism, Vol. III (Colombo, 1973), p.435.Venerable Anandohttp://www.what-buddha-taught.net/Books ... HE_WAY.htm
"Intentionally bring up the question 'Who?' and notice clearly what state of mind follows. The mind stops! We can struggle to find some intellectual answer; maybe our name comes to fill in the gap or maybe an exalted, inspired idea like 'The Original Mind'. But the Original Mind is not the thought 'Original Mind'. Buddha-nature is not the thought 'Buddha-nature'. Thinking is just thinking. Thought does not really answer the question 'Who?'"Ajaan Mahã Boowa Ñãnasampannohttp://www.what-buddha-taught.net/Books ... Dhamma.pdf
The Buddho which is hard and difficult to meet
with (as it says in the above passage in Pãli) will then become manifest in one’s heart.
Buddho may be divided into three kinds, as follows:
1. Buddho — means the Buddhas who arise in the world; but they arise only very rarely,
and only one at a time. In this meaning, Buddho is rare and hard to meet with.
2. Buddho — means the arising of the pure (parisuddhi) of all the Sãvakas, which is also
rare and hard to meet with.
3. Buddho — meaning the “Buddha” inside ordinary beings who are under the influence
of kilesas. This kind of Buddho is common and so not hard to meet with.
How is this so with the third kind? It is so, because this “Buddha” is always supervised,
covered up, oppressed and compelled by the kilesas, tanhã, and ãsavas, which will not let it
show itself and be free.
When the citta or knowledge is of this kind, it is not able to get free and raise itself out of
the mud and mire of the kilesas, taõhã and ãsavas. How then can this Buddha nature come to
know the truth of all things, which is the way of cause and effect in all sabhãva-dhammas?
When this Buddha nature is going to examine, investigate, clean and correct its state by
training all the time in virtue, gracefulness and goodness, it is necessary to start with, that one
should be in a suitable social environment with a wise and learned man (a Teacher) who is
skilled in the ways of curing one’s self to enable one to get free from one’s obstructing
difficulties (upasagga), whether they be internal or external.
"When one thing is practiced & pursued, ignorance is abandoned, clear knowing arises, the conceit 'I am' is abandoned, latent tendencies are uprooted, fetters are abandoned. Which one thing? Mindfulness immersed in the body." -AN 1.230