Can you have some handrails mounted into the floor?
It might sound weird, but if there's not a wall right next to the shower to drill a grab bar into, what about a floor to ceiling pole? As long as it's bolted into the floor and secured into a beam in the ceiling.
or, temporary solution...one of these (altho, if it were me, I'd want something securely bolted to the floor and into the structure in the ceiling)
http://www.amazon.com/Standers-Secur...=cm_cr_pr_pb_t (read the reviews)