Final worklow, with memo.cash as example:
user visits memo.cash. scans CashID code, logs in. memo.cash create child keys, user use CashID to sign them, memo.cash can now post for user
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.