I did something similar. I damaged the 5V supply IC on the OSPi due to an error connecting things. I also wanted a way to power the Pi during short power interruptions to prevent SD card corruption (had it happen several times). So I removed the power supply IC and fed the Pi 5V from a small 12V to 5V converter and fed 12V through the network cable from my wiring closed. 12V supply is powered by a UPS. Was able to find off the shelf parts – wall wart for 12V, power injectors for the ethenet cable, and 12V to 5V regulator. Works great.