I was able to put together an ugly cable between the DB15 and the DC barrel connector and the DB9 serial con on the finish gate. Success! I can power the finish gate through 100 feet of CAT5 cable and the communication is working just fine between the PC and the finish gate.

All that's left to do now is to draw up some end panels with cutouts and fab those - ponoko maybe?

Then, all I have to do is wait for next January! :-)