This came up again because I wanted something fine grained to identify chunks for interactive editing. I'm actually thinking I want to be finer grained that the original design of Purple Numbers, more like the phrase or sentence level, but I'm not sure yet. My thinking is that if it is fairly fine-grained, then edit conflicts will be rare. Each person will tend to be editing a different part of the document, making OT simple in most cases.
I expect more conflicts when doing bigger refactors where you are moving the blocks around, and that conflicts within a current block will be rare. Even when it happens, I want to keep each user editing their own buffer and try to resolve conflicts with a more merge-like operation at the end. I also think this will mesh better with the Federated Wiki ideas that Ward's group in inventing.
If we do this, I don't like the idea of creating some non-card object to deal with. Instead, I'd say it should be a virtual card identified by its relationship to a longer card. (And ultimately we should make it easy to graduate such cards into real cards.)