Yep, your only option is to crack into the case and get to the internal VGA connector, and use that. It's quite possible--I took a 333 apart yesterday, and there's definitely a connector sitting in there--and I've known people who did that when the internal monitor fried, but it's not pretty.
You could, if you don't care about the apperance or using the internal monitor, probably drill a hole in the case and feed the VGA connector in through that, to keep it a bit cleaner.