So I'll be putting in an antique claw foot tub. Today I found a cool, solid brass- heavy- antique gooseneck 3 bulb (I think this is what you call it) has a hot & cold handle -tub mount fixture. It does not have a diverter for shower. I'm wondering if there is a way to put a diverter and pipe for shower head just behind the fixture or just behind on the outside of the tub?If that makes sense. The water pipes in the wall will go straight across to the back side of the fixture. This house is over 100 yrs old, I haven't torn into the bathroom yet, but near as I can tell the drain pipe comes straight up through the floor.

if you can find a similar plain ol' valve, you could mount the outlet of the faucets to a common chromed pipe. put the new valve between the outlet pipe and the tub spout. when shut, it's a diverter, and the water will go against gravity to the path of least resistance, up the vertical and to a shower head.