Well, I considered this for a while, then I ordered a cross-draw holster from October Country. I have a couple of T/C Patriots in .36 and .45. I also have a T/C Hawken in .45 and a T/C Seneca in .36. Sounds like a good fit right? When the holster arrived, I holstered one of the Patriots, then put my possibles bag and a single powder horn on my left shoulder to hang on my right side. Just walking around the house, I found it cumbersome. If you want to sling the long gun, there is no place for it to go. The other issue I had was that in drawing the pistol, the ramrod would sometimes catch on the holster and get pulled out of the gun. Also, we only have two hands, so if you are going to shoot the pistol, you kinda need a place to put the rifle.
All in all, I think it is an interesting idea, but needs further refinement. I think perhaps a specially made possibles bag that incorporates a holster for the pistol might be in order. Also, Patriots are not hunting pistols, so are perhaps ill suited to this application.
I will likely use my holster with the Patriots and possibles bag, but probably will not take it as a backup gun for the rifle. Less cumbersome and likely almost as quick would be a couple of QuickShots for the .45. Squirrels and rabbits I might hunt with the .36 will just have to wait until I can reload as there are no QuickShots being made in .36 to my knowledge.