Enjoyed your post. Thanks!
I ride an R1150GS and understand your dilemma completely. The GS is an excellent road bike and a "jack of all trades," but one has to learn their limitations on the bike. I did a similar stunt to yours this summer by trying to ride a jeep trail in the mountains which quickly deteriorated to large rocks and sand. Bounced the front tire off a sand-covered rock and dumped the bike. Not sure a set of TKCs would have been better than the Tourances on the bike for the situation. Picking up 580lbs at 10,000 feet is a little strenuous. I actually had to spin the bike around on the cylinder head to get it facing downhill so I could lift it. Anyway, not having anything to prove, I turned around and went home. I also had water and a bunch of other gear, but no spot or cell coverage (yes, I had my bags attached, if I took that ride again I'd leave them off). It was daylight and there was a camp about 1.5 miles back down the trail.
I used to ride a Honda TL250 up in those mountains, which, other than the small fuel tank, was about the perfect bike for trail riding - but I had to put it in the back of the truck and haul it to the trailhead. The GS does a LOT more but is nowhere near as fun off-road.
Be happy with the big pig, but realize the limitations.