Any current wallets support this? (I am aware of bitmessage, but that is a different blockchain. I am only talking about the btc blockchain, not alts).
You could use the OP_RETURN bytes to do this but as there are only 80 such bytes I think that would make it pretty difficult to add anything other than a very small encrypted message.
The point of Bitcoin is not encrypted messaging or the like - it is just an open, distributed ledger with no central control.
I am thinking of just an encrypted token for use by an OAuth Provider. Not sure how many bytes that would be. I would have thought that one of the wallet apps would have taken advantage of this field by now.