I'm sure the 2 shops and the dealer said not to because it's not a propper application in their repair book minds. Never mind the fact the MFGrs have been omitting gaskets for years now and using RTV instead.
The RTV can't hurt anything, and at worse, it will just fail. If nothing else, it's an inexpensive fix and your only out $4 if it fails. The big concern would be having a leak over a long period of time and pitting the mating surface of the head.
That is infact one of the times I used RTV on a header. A gasket just couldn't seal up the pitting.
It's your Jeep, do as you wish