Yes, GBE does use all of the wires.
If you want to add POE, look for a MIDSPAN injector and you don't have to modify the switch. POE is still a little crazy.
Your right, there were and probably still are some 100bt injectors that use the spare pair without a signature and there are some "proprietary" formats that even boast 100 W.
They raised the voltages to 57 V DC and used both pairs to increase the current and lower the voltage drop. Furthermore the signaling is two tier. One for POE and a second to address POE+