Remember that there are two forms of PoE. One is the full architected
802.3af version, which is 48V with a handshake to establish things like
current requirement, and then there is the simpler "passive" PoE which
just pushes typically 12-24V over the power pins (+ is 4-5, - is 7-8).
The PoE pins on the BBB (if I remember correctly) are not used as PoE
and are either grounded or connected to ground by a resistor, either way
no use.
However you can get PoE injectors such as:-
http://www.rfelements.com/en/products/power-over-ethernet/passive-poe/
The one on the left would be used to power a BBB and the one on the right
would be used to inject the 5VDC into the cat-5 cable.
However be careful as voltage drop across cat5 is bigger than one might
imagine, so you would be better putting 12VDC into the injector and then
connecting a 12V - 5V converter (either a car USB adapter or one of the
little DC-DC converter modules) and pluging that into the BBB as it will
be a "better" 5VDC.
David