One tactic would be to make a haskell eDSL for making the same AST as ParseBP

e.g. using OverloadedLists and OverloadedLabels extensions, this kind of thing could be possible:

polym [[#bd, #sn], [#cp, #sn, #arpy], #snare]
"{bd sn, cp sn arpy, snare}"

polyr [[#bd, #sn], [#cp, #sn, #arpy], #snare]
"[bd sn, cp sn arpy, snare]"

The above still wouldn't let you mix tidal expressions like rev in with the maxi-notation though, so doesn't buy very much, if anything.
The good thing about the AST is it can track the relative 'size' of a subpattern in steps. So maybe the real task here is getting Tidal tracking the numbers of steps in a pattern again.

The work on Epic by @JeffreyBenjaminBrown likely relevant here http://blog.tidalcycles.org/some-experiments-reified-duration-multi-parameter-sequences-in-a-single-string-and-chord-sequences/
https://github.com/JeffreyBenjaminBrown/Tidal/tree/epic