The only problem i have is when someone breaks a truce without warning...
Just for the record, even that
is not cheating (might be against your moral rules, but it's not against the actual WL rules).
Two more thoughts:
If PMs are disabled I think it's still okay to have truces; you'll just have to deal with the fact your opponents can read along. (I do have a problem with people talking in private in another game.)
While I've never actually done this, it is possible to discuss privately in public chat, Google "public key cryptography". While technically not cheating, in practice I would consider it as such.