We stayed at CR in Dec 2007 and didn't get the HA roll-in shower that we had booked (CM misunderstanding), but they do have tubseats available on site if the one in the roll-in shower isn't adequate, just call housekeeping and they will bring one right away (they have them with and without backs).
We had a roll-in shower at WL and the seat would have been adequate for my mom if she had traveled with us. The one at CR may be different, though.
If you haven't talked with a CM at Special Reservations, I would make sure I did soon, because I understand CR only has 2 rooms that have the roll-in showers, and the CMs at CRO might not understand what you want (based on my experience).
Suzanne