For the single-gang LCD, we do indeed use 3 of the 4 pair, as electron noted (He is fast becoming your "in-house" expert on the product). Two pair are used for communication, and one pair for power. That leaves a spare pair.
We have also tested some of the flat CAT5 cable, in case someone wants to "hack" a new cable pull, under the carpet and up the wall....it worked, although the flat cable produced more of a voltage drop, so the distance might not make it to 340 ft. I have heard of some flat CAT5 cable that can be drywalled in, and would be curious to test that too.
For the real techies out there, we have also included an I2C network. This would allow others to put sensors on the LCD side, such as temp, light, buttons, etc. We put this in primarily for OEM uses.