Yeah, I saw those babies at $1K each! Then probably $2K after trade in for another subal housing plus whatever camera Nikon comes out with that has 1080p...Yikes. I got 2 kids in college and another going in a year. Plus I still like to travel. I need to hit the lottery.
Maybe all of us should chip in a buck and get some tickets 
I do like the idea of WB to the filter on the surface then adjusting in post for the lights. The L&M lights I had kept shutting off and that technique would have given me a lot more usable film I think. Wags stuff always looks good. I'd be interested in finding his posts with the filter technique.
He has a few posts (well more than a few) on white balance and through the threads. They was a good one discussing the cuddle technique (kind of funny one). But the one I was thinking of I could not find just now. Was good for the situation you described (no ability to access WB in camera.) Basically it was something like "Guess how I did this guys," for some video. What he did was set white balance at the surface (for some reason I recall against the side of a white/grey van) and just dove that way. Not sure if there was a limit as to when it was not as effective.
Correct me if I'm wrong but it seems that the high ISO performance of the vdslr's just gives you a better less noisey picture with a filter in low light than a camcorder is capable of right now. I'm happy with my D300 and my "lark" like video. By the time I'm ready to do the next big $8K upgrade it will be a different ball game by then anyway. I can probably find a used Ikelite housing before the next trip. Cheaper toy.
I have not found any comparisons among cameras (i.e. Canon HC9 vs Canon 5D Mark II) and have not had a chance to dive with the 5D , but from what I have shot above water and in darker conditions, it does a real good job.
Some discussion on the 7D vs 5D video quality issue more for some background and this thread 7D footage for low light. There were also some other good discussions of pros/cons, but could not dig them up. Aliasing and jello affect are the big issues and the vDSLR, and some ergonomics, but also how the camera (5D) is being used alot despite the issues.