Yes you can encrypt/decrypt messages using your Memo wallet key. Encrypted messages can be stored in OP_RETURN. e.g. https://github.com/memocash/mips/blob/master/mip-0004/mip-0004.md
Wouldn't ECDSA encryption for another recipient require exposing your root public key (not just the hash)? Sticking it in OP_RETURN makes sense IMO.
I haven't looked into it in awhile, but last I remember ECDH can be used
