What's she like in a shower?
Or can you put very little water in the bath and then see how she goes.
Or even put her in the bath with no water, wet her with a facewasher and soap her up and then let her rinse herself with a few cupsful or icecream containers?

We have a shower-hose thingy that fits on to the bathtap and turns it into a really gentle hand shower - one of those might be really useful here too. Ours was about 10 bucks from safeway.