I’ve recently had an open mike problem, fortunately no big deal as I was on the ground at the time. However I was thinking about a way of dealing with this in the air on an emergency basis.
I’ve got 2 x GNS430s with a KMA20 audio panel. When I listen to ATIS in the air I often set ATIS freq on box 2 and manually switch com 2 to “phone” whilst working with box 1. That way I can listen on both freqs at the same time but only transmit on box 1.
If I get a stuck open mike I thought I could set the working freq “X” in box 2 and also set X as the standby in box 1. The active freq in box 1 could be 123.450 and I would be transmitting open mike on that freq. I could then use the flip switch in box 1 as a PTT, flipping back to 123.450 as soon as I finish my message. I would hear the response from freq X through the manually selected com 2 switch.
Can anyone think of any reason why this would not work?
The GNS430 has open-mike protection. I think after 30 seconds it disables PTT.
The KX radios also stop the transmitter after some 20-30 seconds, but that doesn’t address the more pressing issue of a PTT switch which is jammed ON.
Ted’s proposal should work, I think. He just needs to be careful to not slag off the owner of his airport when flying, because half the world uses 123.45 for stuff like formation comms 
If the transmit disables after 30 secs then I have another idea. I leave the setup previously described. Once box 1 disables transmit I presumably have 30 secs avaiable in box 2? So I leave box 2 manually selected to phone but I rotate the switch from box 1 to box 2, effectively using it as a PTT. I transmit on box 2 then immediately switch back to box 1 (with its diabled PTT), I can still receive box 2 because of the manual phone selection. If that does not work perhaps I could power off box 1 and use the box 1/2 selector switch as a PTT, switching it to box 2 to transmit and then switching it back to the dead box 1. Box 1 cannot transmit because it is off, but I can still receive box 2 through the manual selector.
He just needs to be careful to not slag off the owner of his airport when flying, because half the world uses 123.45 for stuff like formation comms
I’d choose 118.00 MHz, which is typically reserved to ground test sets.
