From memory, the chorus techs were telling me for their wifi project in christchurch, they could power a VDSL ethernet bridge and access point across a couple of copper pairs from the cabinet or exchange. 

 

A quick calculation tells me over 500m using two pairs of telephone wire there is 20 watts of power if they feed it with 56 volts at the source, and it comes out ~48V so can be adapted to work for POE powered devices.