I've envisioned to solve this by sending the change back to the sending address. As sending cards to self is equal to having cards in the first place this is effective and will be easy to parse from the blockchain by the client.
Ie, in PeerAssets Peercoin transaction there would be only one referenced address which is not 1) sender, 2) P2TH derived address.
But yes, this is not written in the paper. I have just forgotten about it while I was writing.
Do you think this is good enough solution? If you agree I will write up the text explaining this and place into appropriate places in the paper.