I have a double wall switch next to the fireplaces: one selects AUTO/MANUAL operation. The other is a simple ON/OFF, which only applies when you are in the MANUAL mode.
While I can't draw the diagram here, the basic idea is as follows: have a small relay operate the actual fireplace connection. The relay is powered in turn by the CC signal or the ON/OFF switch.
You can have simpler implementation by just wiring a manual switch in parallel (shunt) to your CC device. However this will only allow you to force it ON (can't turn it OFF when the CC is closed).
You could add addition switch in series, to force it OFF, thus avoiding the relay, but royally confusing the users in your house. Maybe labeling the switches FORCE ON and FORCE OFF? On the bright side this would still work when the line voltage is out.
Note that the fireplace switches require a fairly thick gauge wiring wince they operate on a really low milli voltage. If the wire is too thin it will not work or work erratically.