Even so, the non-recipient participants/users will have to trust the recipient of the message to show the correct message, since they wont be able to decrypt it by themselves.
Sounds reasonable enough if used together with the forum's trust system but involving trust is kind of a drawback.
That's the reason I suggested to use more than two members. So authenticity of the revealed message can be validated without error from one member. It's just information and I don't think anyone who are on the forum for long time will even try to alter the message.