I don't think it is a bad idea, though a thread like this would have to be constantly refreshed.
What I suggest is, send a PM to the person who posted the summon and you can ask that person what ever you need know to get in contact with his PC guard of an other guard.
I think this might be easier than keeping a thread up to date with all the PC guard.
Sending a PM, or contacting the player OOCly in-game, is a response. A thread, as qwerty says, would be a "far more of a helpful resource than a harmful one." Resources can be utilized, and in this case, I think in more ways than responding to a summon.
Given there are already faction forums managed by the staff, refreshing the thread would be a simple matter of add or removing a Garda from the thread when they are added or removed from the faction forum. This logic really only applies to Vallaki Garda, but as Arawn corrected, there's no active Wachter Garda, and AFAIK, the Port-a-Lucine Gendarmie uniform is worn by Roland, and NPCs. So.. on second thought, there might not be much need for a Gendarmie roster thread.
With just Vallaki Garda as the focus of such a thread, I think it would be manageable enough.