Interesting, agree that for the adv application the weight is not so important as the sturdiness. And tubeless? That's a major advantage.
The Sportster I have now I can't bring myself to chop into an adventure bike, it's just too nice. With the mods it handles OK, but the stock wheels! Heavy as sin, you just know the handling would be much sweeter with 10-15 pounds less at each end. But I just will not spend thousands for lighter wheels/brakes. I've never built a wheel but that's an option and wouldn't be too expensive...
I think if a cheap enough 883 fell into my hands I might see about adventurizing it. If that worked out then maybe a basic 9.5 to 1 1200cc conversion. After my drag racing phase I kind of regret going the 10.5 to 1 level three build. Under 3,000 rpm isn't all that accessible.
My Sportster I think nothing of taking down dirt roads etc. As has been pointed out it works well in loose conditions.
2006 Ninja 250
2012 SYM Wolf Classic 150
2012 NC700X (HonDog)
2008 KTM 990 Adventure