It’s pretty straightforward math! I did an M:N implementation in Elixir via on shamir’s key sharing [1]. As you say, there’s no real standard for how to pack the hash bytes for even basic Shamir’s and so I followed Hashicorp Vault’s impl and tested it to their test data. Multiplexing an entire file over this wouldn’t be too hard, but the index/hash table would be need a good protocol. Wasn’t there some cloud storage systems tried using M:N at one time..

Plus the recombination computation can get expensive over lots of data.

1: https://github.com/elcritch/keyx