I think I have pretty much done what you require using a .lua script. Maybe a little simple, but it appears to work OK for me. (Not too much testing done.)
How do you feel about using .lua scripts in FSUIPC?
I do not have the Flight1 Cessna Citation Mustang, but I tried this on the default FSX Cessna 172SP Skyhawk G1000.
Here it was the standard Com / Nav offsets to do this.
Hope this helps.