dan, I do think you're over analyzing it a bit. Build it, test it, if there's a problem you'll find out, given you have Cat5 wiring to work with you can't go wrong, you have 8 lines you can dynamically configure.
Remember if waste were an important consideration no one would use UTP cabling because only 2 of the 4 pairs of wires up to and including 100BT networking are actually used, the rest are wasted. Gigabit over copper and power over ethernet setups actually use these extra wires, which is one of the reason why 4 pair UTP is required for Ethernet. Cat5 for 100BT and Cat3 for 10bt, the only real difference is the number of twists per inch of cable and that only has noise reducing effects for differential signals (meaning not applicable in your case) Cat5e is nothing more than a high end quality approved Cat5 cable (oxygen free copper and excellent insulation materials)
Personally if it's not network based I prefer cat3, because the fewer twists per inch make the cables a LOT easier to deal with for the thumb fingered folks like me. For what you're doing you could use nearly anything.