I was reading this and I realized that under the stated conditions (two parties who trust each other during conversation; later on attacker gets full access), there is a more practical solution: SSL-secured chat with no logging.

It is not as neat cryptographically-wise, and it does not work with offline message exchange, but I think that for a lot of common cases, it has the same guarantees (confidentiality, authenticity, deniability, forward secrecy)

Or am I missing something?