I've released the #CashID specification (https://www.yours.org/content/introducing-cashid--an-open-protocol-for-authentication-with-optional-e3df11d498e6). Would love to see it on memo.
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.
I'm not sure yet, but probably a custom action like you said. Delegated keys is something I was hoping to start working on in next few months. Once I dig in I'll know more.