Does your controller give a rough schematic of the input circuitry? The ones that I've seen had the "differential" input simply going to the diode side of the optoisolators with an antiparallel diode and a small series resistor. If so, driving it single ended is no problem, though if it were me (and since you already have it wired up with twisted pair) I'd do the grounding of PP- back at the BOB.