It would be pretty easy to implement messaging between two people who can log in to the same bank account. There are plenty of places you can store text typically, labels on payees, your details etc. Or one could use two accounts under the same login: person A logs in and sets the balance on account 2 to something which represents a character in binary. Then person B "acks" that character by transferring the money back to account 1. Repeat. Those characters could in fact be cipher text for full end to end crypto that even the bank can't decipher.