In the control area underneath the fireplace there are two receptacles (plug-ins) labeled for a REMOTE and a FAN; these are wired.
If the receptacles underneath the fireplace are wired ....there is a pretty good chance this is (the key to) your power source.
Check the receptacles and see if they work at 110-120 volts (I would plug in a light).
If the light works, AND flipping the switch makes the fireplace work. Track down the breaker and see if that will work as an ON/OFF switch.
If that works...install an in-line non-dimable X10 module in a place (likely a junction box) that will NOT effect other receptacles or devices that may be on the same curcit.
My lawyer asked me to remind you I have never seen your set-up and cannot be held responceable for my internet ramblings.