Nix master has recently landed changes that create a separation between the low-level representation and a more high level representation.
It means that consumers of the new types can focus on the meaning rather than how that maps to the current 20+ year old format that’s been layered on top of.
Output hashing modulo meta will fit nicely on top of those (rebase incoming). I know you’ll like that one 
Most notable refactor I think is libstore: compute the hash modulo via an intermediate derivation by amaanq · Pull Request #16191 · NixOS/nix · GitHub
I suppose your library is only slightly above the ATerm level?
That’s already good enough for most applications, including extracting the meta field, but if you want to also write derivations, the extra layering becomes useful.