Light & Motion also planned to offer manual white balance via the back door camera restart process, but has found that it just doesn't work. The camera will wb on a blue shop towel, something a manual white balance wouldn't allow. I expect, even if Amphibico offers it, that it won't really work either.
This leaves the A1u as the clear choice. Light & Motion's bluefin A1 will access the custom button, which can be programed for manual white balance, making this a very attractive option.
Sea & Sea's housing would also be compatible w/ the MWB, but it strangely didn't access the telemacro button.