I would like to write the correct form of the following sentence:
He played the guitar in/as accompaniment to/of the choir's chant.
Which is the most correct, and why?
Accompaniment to is more widely used than accompaniment for. (See this Google Ngram. )
Merriam Webster's example sentences for accompaniment are:
She sings without musical accompaniment.
A nice tie was a fine accompaniment to his new suit.
This dish can be served as an accompaniment to most meat main dishes.
This wine is a good accompaniment for spicy foods.
She studied Italian as an accompaniment to her classes in art history.
The choice of as or in preceding the words accompaniment to seems to be less well defined. (See this Google NGram, with as accompaniment to slightly more preferred.
My preference would be:
He played the guitar as accompaniment to the choir's chant.
(I [being an editor] would also ask if you might not be able to rephrase the sentence to: He accompanied the choir's chant on his guitar.)