When logging into Busy, DTube or DLive through Steemconnect, they keys are checked in real time. I can imagine a similar tech can be used to check memos during a transaction.
Another option would be to change the Steem blockchain to require an owner (Master) key to transfer funds. It will be inconvenient but way more secure than it is now.
Interesting. Cheers. I'm sure some JavaScript was in place to check the memo field for something that resembled a key, no-one has confirmed this so far though so I could have been dreaming :)