I don't know about the serial port, but here are some other options for power:
1) If you wire your expanders normally with cat5/6, there will be 1 extra pair of wires. 1 pair for power, one pair for data A/B and one pair for data A/B return. The 4th pair is unused. If you use the data bus hub this pair is easily accessible. Why not send power to the expander this way.
2) Hijack the data bus power supply. Each expander draws power from the power supply. Probably NOT a good idea for much of a current draw, I'd rather keep the data bus power separate. If anything goes wrong with the data bus, BAD things happen.
3) Grab an expander and check the pins. You can see if any voltage is available at DTR(usually pin 4), but only ELK can tell you if their chipset can provide power(assuming there is some voltage somewhere). I guess you can grab power from other pins, but they are going to be intermittently powered depending on the data being sent/received.