Hmm, perhaps PoE is strictly 10baseT and 100baseT standard? That way
>they can send the DC through the unused conductors.
You don't need any "unused" conductors.
Consider a simple case of 2 pairs, one for transmit and one for receive.
You can also send DC over the same wires by making the receive pair positive relative to the transmit pair. The downstream station picks the power off the center taps of the isolation transformers.