Only caution with this approach is that the cigarette lighter is switched (so will be off when the car is off), but normally the ethernet network is still up and running. So you may want to use a non-switched power source... the little NETGEAR GS105Ev2 I have takes 12V DC directly, so should be somewhat straightforward to tap in somewhere.
[edit]I guess if you use a coupler for normal use and the switch just for hacking sessions, it should be ok.. no need to hardwire it then![/edit]
You can do 12v DC from the OBD port underneath the dash on the left hand side.