Here are the actual Pali sentences:
Kayasakkhi Sutta: Bodily Witness
"'Bodily witness, bodily witness,' it is said. To what extent is one described by the Blessed One as a bodily witness?"
"There is the case, my friend, where a monk, withdrawn from sensuality, withdrawn from unskillful qualities, enters & remains in the first jhana: rapture & pleasure born from withdrawal, accompanied by directed thought & evaluation. He remains touching with his body in whatever way there is an opening there[see AN 9.35]. It is to this extent that one is described by the Blessed One as a bodily witness, though with a sequel.
"Kāyasakkhi kāyasakkhī" ti āvuso vuccati, kittāvatā nu kho āvuso kāyasakkhi vutto bhagavatāti.
Idhāvuso bhikkhu vivicceva kāmehi vivicca akusalehi dhammehi savitakkaṃ savicāraṃ vivekajaṃ pītisukhaṃ paṭhamaṃ jhānaṃ upasampajja viharati. Yathā yathā ca tadāyatanaṃ tathā tathā naṃ kāyena phassitvā viharati. Ettāvatāpi kho āvuso kāyasakkhi vutto bhagavatā pariyāyena.
Gavi Sutta: The Cow
"When a monk enters & emerges from that very attainment, his mind is pliant & malleable. With his pliant, malleable mind, limitless concentration is well developed. With his concentration well developed & limitless, then whichever of the six higher knowledges he turns his mind to know & realize, he can witness them for himself whenever there is an opening.
Yato kho bhikkhave bhikkhu taṃ tadevasamāpattiṃ samāpajjatipi vuṭṭhāti pi. Tassa muduṃ cittaṃ hoti kammaññaṃ,mudunā citte kammaññena appamāṇo samādhi hoti subhāvito. So appamāṇena samādhinā subhāvitena yassa yassa abhiññāsacchikaraṇīyassa dhammassa cittaṃ abhininnāmeti abhiññāsacchikiriyāya. Tatra tatreva sakkhibhabbataṃ pāpuṇāti sati sati āyatane.
I think "opening" is Ven. Thanissaro's interpretation for sakkhibhabbataṃ... but not sure.
Sakkhi = face to face; before one's eyes (witness).
bhabba = able; capable; fit for.
ta = (demonstrative pronoun) that; ta + ṃ = "that thing."
So, I think sakkhibhabbataṃ = the thing that one is able to witness... or "opening" in another word. This is just my guess.
I don't see how the "opening" really fits in with the sentence from the Kayasakkhi Sutta, though... the body would be the "opening" I guess, but I don't think the Pali sentence really says it in that way (from what I can figure out).
I think that was misleading for Ven. Thanissaro to refer to one sutta from the other (at least without a footnote to explain the connection), unless I'm missing something. My interpretation above could be completely wrong.