Pretty much all the early Buddhist schools featured sunnata / sunyata as an important part of their teachings.
In particular, the Sarvastivadins (before they were actually known as such) referred to themselves as the "sunyavadins" when they argued against the Pudgalavadins' theory of the "pudgala". They feature sunyata as one of the sixteen aspects of the four noble truths (four each). Along with anatman, these two were the only aspects that were applicable to all four truths. (The others like anitya being not applicable to the truth of cessation for instance.) It also featured heavily in their three samadhis system. They also have a few sutras that use "sunyata" as synonymous with dependent origination, such as the "mahasunyata-paryaya" and "paramartha-sunyata-paryaya". Neither of these sutras are found in Pali. However, they played an important role for several others schools, it seems. (eg. Sautrantika, and the *Satyasiddhi.)
Whenever the Mahasamghika sutras use "sunyata", it is often part of a set like this: The four immeasurables, loving kindness, compassion, sympathetic joy and equanimity; then the three samadhis, emptiness, nothingness and the signless. (This latter form of the three samadhis may be older than the form emptiness, intentionless, signless.) There are suggestions that in fact the four immeasurables are used as the samatha basis for entrance into the three samadhis. ie. cultivate loving kindness up to deep dhyana, then turn to contemplation of not self. This pattern is similar to the sutra on Purification of Almsfood, and the Sunakkhata sutta. (From memory!)
The *Satyasiddhi Sastra also heavily features sunyata. However, it has more of a Mahasamghika turn to it. It is used not just as "empty of self", but a generic "empty of ..." So, they used it a lot as a synonym for nirodha / nirvana, the absence of (empty of) defilements, and / or absence of deluded conceptualization.
Whatever the case, I am not sure what you mean by "trappings" of Mahayana. If you like the idea, just use the idea. No need to take other parts like the bodhisattva-theory or whatever. They can work independently, so to speak.
In recent decades, scholars like Warder and Kalupahana have argued that Nagarjuna's Mulamadhyamaka Karika is based on the Agamas, and not necessarily Mahayana at all. Actually, Yinshun already said this decades before that! It's entire content is consistent with readings of the Agamas, though not necessarily the Theravada interpretation. But after all, the Theravada was just one school. Other schools had their own take. In particular, Nagarjuna's association of sunyata with dependent origination and the middle way is perfectly in accord with those Sautrantika sutras mentioned above. Perhaps the Theravadins lost these texts at some point. Hard to say.
You'll just have to look beyond the usual stuff that is found in Pali sources, and possibly beyond Sanskrit sources, too.
Full disclosure: My PhD is on sunyata in the Prajnaparamita, following it's development through early and mainstream Nikayan Buddhism.
My recently moved Blog, containing some of my writings on the Buddha Dhamma, as well as a number of translations from classical Buddhist texts and modern authors, liturgy, etc.: .