The real "power over ethernet" system is complex because it uses high voltages @ low current to transfer a lot of power (>10Watt) with minimal loss over long cables, and also use a complex "nature of the receiver" polling system to see if the receiver is of the right "type", and what its power requirements are, thus real power over ethernet is a system, that by nature requires a lot of expensive components.
But if not adhering to any standard is not a problem, and you only want to transfer small amounts of power over relatively short distances (less than a hundred meter) you could just use a few unused wires or the RJ-45 connector to directly transfer power to the powered device from a central power supply.
The R-PI only needs 5V at a few hundred mA, so the voltage loss from a short cable is negligible.
My solution would be to use the wire pair for pins 4 & 5, disconnect them from the ethernet connector at both ends of the connection (RPI and ethernet hub) and use that pair to transfer 5V from a central power supply to power all the R-PI's connected.
If one pair isn't enough you can repeat the same with pair of pins 7 & 8.
Take care not to reverse polarity, or you will destroy the R-PI.