If I want to use a hash that isn't yet in multihash's registry (or will never be because reasons, but I nonetheless want to use it), what do I do?
I could make a forked copy of the registry where I allocate my own number to the hash, but, I have to guess very well what numbers will not be used by future allocations in multihash. If I guess wrong, I might have to go through every hash I've encoded with my forked registry and re-encode them, which could be very onerous if they're spread across s3, glacier, cold storage, etc.
If there is a well-defined set of numbers that are set aside for personal / non-sanctioned use, I have the same problem when examining hashes from other parties, because they may have chosen a different number for a particular hash, or we may have conflicts. Likewise, when a hash "graduates" from the personal space to the official mapping, all the old hashes are now encoded "wrong": they'll keep working, as long as you ensure the hash keeps a number in the personal space, but now you have to choose whether to, when making new hashes, to use the official number "for consistency (with the standard)" or the personal one, also "for consistency (with your existing hashes)".
It looks like every hash in multihash has a number (like 0x13), a name (like "sha-512"), and a description ("The SHA512 Cryptographic Hash Function") [0]. I suggest letting people use e.g. "{name}:{len}:{hash}" if they choose to make the tradeoff in favor of readability / portability (with lower performance and marginally higher storage cost).
Then when those hashes get read by some other person's copy of multihash, they'll either map the {name} to the local number (if if it's different from the number of the multihash library that encoded the hash) or if the {name} doesn't show up they can throw an appropriate error. This is forwards compatible and I think will allow more people to be comfortable adopting multihash, because they won't have to think so much up-front about how to encode hashes that aren't in the registry.
[0] https://github.com/multiformats/multihash/pull/41/files#diff...