I'm definitely down to do this again with a little notice. DRZ could use a workout and it'd be fun to see what else Knoxville has to offer.
The ride from San Jose to Knoxville is a slog but you have Metcalf in your backyard. From SF Knoxville is definitely the closest and the ride is not bad, in fact Knoxille Berryessa rode is a blast on a dual sport.
I did Knoxville on my 1200GSA many years ago, with Metzeler Tourance tires. I took the main loop from/to Knoxville Rd, about 10mi. It was OK and I did not drop the bike, but I would not do it again, just no desire to push my luck. The GS can be picked up if you are alone, I did it a few times, but it is a pain to do. No problem with 2-3 people.
Of course I did ride all the way from San Jose and back, no problem on that bike. And Knoxville Berryessa Rd is very nice, except that on one of my group rides another guy dropped his 2 day old GS in one of the water crossings by accelerating.