Yeah, I could see a passthrough thing, where you had an edge connector (or something), that was the 8-pins for networking and two for +5V and the backplane was just a +5 rail, and the rj45 jack right next to each slot. As a zero experience with PCB design kind of person, this seems relatively simple. You could make a 'one blade backplane' with a PoE kit for the single/small number of devices case where PoE makes more sense.