https://multiformats.io/multihash/
It fills in some of the blanks in your "prefixing the hash with the value of the enum for the hash" step.
However, there are a two general problems:
* The spec is not done, and it doesn't look like much has been done.
* While I agree with the FAQ that agreeing on a set of hash functions is possible, the format only allows 256 possible hash functions, so it can run out of space, especially since there can be multiple formats of the same function (BLAKE2B and BLAKE2S, for example), and especially since they want to allow non-cryptographic hash functions.
Then specifically for my use case:
* There is the problem brought up by AdamN [1]: if multihash is supported, an obscure hash may be supported, and that may cause problems.
* As an extension of that, once a hash function and set of settings is supported, it's supported forever, so I want to be more picky, and an enum allows me to restrict what is usable.
* By using a 16-bit enum, I have more space to grow.
* And finally, by using an enum, if my software encounters a repo with a enum value greater than all of the values it knows, it knows that that repo needs a later version of the software, since I will only add enum values.
Location will actually be done by prefixing the hash with the value of the enum for the hash function/settings pair that made the hash.