You could achieve this without any permanent alterations or opening the wall by putting a submersible pump in the tub and hooking the output up so the output fr

You could achieve this without any permanent alterations or opening the wall by putting a submersible pump in the tub and hooking the output up so the output from the pump goes into the shower head pipe. The water would come out of the tub faucet. You can even hook it up through a diverter used for a hand sprayer and the shower head would still be usable. You should be able to get the parts to make an adapter for the output of the pump to the shower head at any hardware store. Use a length of PEX tubing to go from the pump to the shower fitting. Make sure to use teflon tape on all the fittings and plug it in to a GFCI protected outlet. Make sure not to actually turn on the tub while this is hooked up or you could damage the pump.