Re: Projector Wall
I took a look at the video you posted, and it looks like the problem is that Xgl is not sending Expose events to its clients when they're moved across the Xinerama split. I don't know how Xgl is moving around the child windows, but you may need to modify it to send Expose events to its child windows when they're moved across the parent server's Xinerama screens.
As for other ways to set up that configuration without Xgl, I'm afraid your options are limited since the X server, as you surmised, does not support Xinerama plus Composite. SLI Mosaic is designed specifically to solve this sort of problem, but I can understand if that's outside your budget.