One of the properties OTR has is deniability after the fact: closing a conversation reveals the signing key. A similar technique could also be used with a microblogging protocol both to make past posts repudiable and to repudiate entire identities. For expiration, one could have a daily signing key and reveal it after expiration. For identities, just leak the master signing key. It would help even more if broadcasts have variable lag and aren't 100% reliable.
Low reliability means that even if one has evidence of a particular post, they could never prove that the signing key hadn't already been revealed.
A social network for the 19A0s.