Based on your comment about your head touching the ceiling while standing in the tub, I would not recommend spending the money to install a shower head over the tub in a traditional fashion. I think that a potential homebuyer will most certainly notice that something is not right. In addition it will be tough to run water lines through the wall without messing up your tile (and even more difficult to find someone willing to guarantee that they won't break the tiles in the process). Anyway...why not consider just changing out the tub faucet? They offer tub faucets with handheld sprayers that have all sorts of adjustable spray features. To help present the idea a little more maybe consider purchasing a thin teak or solid surface seat to span the width of the tub. While I doubt most would sit on the seat to wash themselves, it could be done, this scenario would also come in handy for a parent and child. I hope this helps! I have other thoughts, but they would involve an intensive/expensive remodel. (Take a look at an example of the tub faucet in the following link http://www.moen.com/eva/brushed-nickel-two-handle-high-arc-roman-tub-faucet-includes-hand-shower/_/R-CONSUMER%3AT944BN