Looks like a good spec! There is a MIP for delegated keys in the works that could potentially use this. https://github.com/memocash/mips/blob/master/mip-0003/mip-0003.md
Just to be make sure I understand this correctly:
any memo-implementation creates child keys, puts a request on-chain that if signed by the users master key, links it to user identity
If I understood this right, then my only complaint on memo.cash (that I am not in control of my own keys) is finally solved - and with a userfriendly design!
with this, you can use your vanity address as your identity. your posts won't be made by it directly, but linked to it. memo would have their own child keys, which you sign to approve.