Uh... I don't quite see how what you described could be an A720-only problem.
Of course, it's entirely possible that the A720 might have different (and possibly worse) exposure algorithms - that's one thing.
But if a scene contains a high contrast (very bright *and* very dim parts), any camera that outputs 8-bit color will either underexpose the dim parts or overexpose the bright parts. That's why HDR combining is done.
I don't know if what you asked (setting a threshold of overexposed pixels) is possible in CHDK, but I really do suspect that, no matter what you do, you'll be left with an underexposed background if you want high dynamic range on the t-shirt.
Have you tried using spot metering? That might be faster and/or more fine-tunable than using the exposure adjustment, if you practice finding the best spot to meter your photo at.
