Is there any problem with piggy-backing a headset onto timing wires, especially coming from the finish? For example, on the same line connected to the system B photocell and timer?

Could this cause false finishes?

Would it be better to have a seperate line for the comunications?

No there is no problem if you run comunications in your timing wire, we do that in track cycling using voice amps in each point we want to comunicate and we conect the operator head set in the Alge S4 or in the Alge timy here is the link of the voice amp.
http://www.sportstimingusa.com/alge_sv4.htm

Thanks