palatization of y- from *ga-

This is a rather negative answer, but the full explanation would take several pages

Does the prefix surface anywhere in a form that would rhyme with ja- or at least ya-?

I have been unable to find any words that began "ga" that developed to "ja"- but absence of evidence is not evidence of absence.

There is evidence to the contrary in the word "gather". In OE, this was recorded in "gegather" (brought together = united) and this appears as

c725 Corpus Gloss. 512 Compactis, gegædradon,

but in eME, it appears as c1175

Lamb. Hom. 147 An is..þet faire icunde þet is igedered bi-twene saule and licame.

Note how the first "g" changes to "i" which was pronounced /j/: this was as the first (softened/unemphasised) "g"[1] is replaceable by the OE letter "Yogh[2] + e" but the second, hard "g" is not, and it is the yogh + vowel but not "a", because would have created the hard "g" of "gather".

However, it appears that words that began with, or contained ia- common in Latin - were all pronounced /dj/ and this is where the seems to originate.

That the "j" appeared in the first place, and yogh was lost, is most likely the Norman-French influence and also that of Church Latin and the transition to Modern English

The situation was further complicated by the letter "J", which was a comparatively late modification of the letter I, pronounced /dj/, akin to that of modern English di , de , in odious , hideous, (and Latin - tending more towards /j/ - iactus, iam, Iouem, maior, etc.) and developed to /dʒ/: Compare jump, Jove, Major).

The introduction of "J" then gave those words the initial "j".

[1]This is supported by Modern German Dialect in which, for example, "gegangen" (pp to go) is pronounced /j/egangen See Amazon music "Als wir jüngst verschütt jegangen waren" https://www.amazon.de/-/en/dp/B007WHAW8Y

[2] OED: (The name of) the letter ȝ commonly used in Middle English to represent the palatal semivowel /j/, the voiced velar fricative /ɣ/, or the unvoiced velar fricative /x/.The letter ȝ developed from a form of g in Anglo-Saxon manuscript writing (ᵹ) and became distinguished from the continental form of the letter (the ancestor of modern lower-case g) in the early Middle English period.