Well as I explained in my previous post she's my 2nd guess, but the reason why I don't think it's her, is because she didn't do anything at all. She just snored through the entire event, not even uttering a single word and she didn't even use her magic when the traitor captain tried to flee. This would have made her the easiest person to impersonate as the only thing Liar would have to do, was taking on her appearance and sleep. Liar was quite proud of his performance, but I don't think there is much pride in impersonating someone who sleeps all day.
When you try to impersonate a mage, there are three things you need to copy: his/her magic, looks and personality. Liar's magic seemingly allows him to do the first and the 2nd is something that seems to a widespread ability as it has been shown before. The most difficult part would be to impersonate the personality, but someone that is asleep all the time can hardly be called difficult. Hence I believe it's Lil as he was quite talkative and even restrained the traitor captain. This would also have benefitted him greatly as by capturing the traitor, it would make people less suspicious to the idea there's another traitor and that it's him. Lil's performance lived much more up to what Liar was boasting about than Dorothy's.