Interesting thread! I have a set of UST rims. I have also been fantasizing about buying a fast road tire (Pasela non TG??) and make it tubeless for long distance riding (rando/touring.) A caveat is that tires would need to be 1.5" (37mm) or wider for these rims which actually makes sense for long-distance comfort.
BTW, I ran these rims w/ UST-specific tires for years on my MTB without a single flat in thorny and rocky AZ. Actually, I did get ONE flat once riding down Mt. Hood in OR. A pointy root caused a 1 inch gap with a hole in the middle. It wasn't difficult to remove the tire, although it was a little messy with the white Stans sealant, but a small rag worked fine. A tube was good enough to get me back down to the trailhead. I remember the Stans sealant kit was pricey, but I haven't checked lately.
Hmmm... Got me thinking!