I dunno if there is a trick to doing it, but not that I've found (not that I've tried hard. I just know if my computer gets accidentally disconnected my monitor goes into disconnected mode).
I think it's a bit silly the programmed the mac to do that honestly. I don't really see the reason for it (if some one knows, I'd love to know why they made the mac not connect to a monitor when on battery). Only thing I can think of is that it may eat battery quicker but you know what, so does Sims (I get like maybe 4 more like 3 hours of battery playing Sims 3 on my 2010 13" MBP with it's 10 hour rated batter). I don't see them outlawing me doing that with my MBP.