I did something similar but not completely meeting the POE specs. I bought a set of power injector adapters that allow power to be put on the unused ethernet conductors at the router and broken out at the Pi. Then got a 12V to 5V regulator board for the Pi end. Use a wall wart at the router plugged into a UPS. I damaged the power supply IC on the OSPi so I removed it. Now I supply power to the Pi power port. Eliminates SD card corruption for when I lose power which does happen from time to time. Would be great to have at least a jumper to disable the on board power supply. Someone probably makes a breakout adapter that meets POE specs if there was a jumper or you disable the OSPi power supply.