I have indeed smoked/cooked a boneless prime rib on my Weber Genesis.
When lower-heat or indirect cooking, I've found the most consistent results having the food over a turned-off side burner. (My last two grills have had burners side-to-side and not front to back.) I used a heavy-duty foil wrapped packet for the woodchips (Applewood, soaked for about 30 mins), on the burner furthest away.
As to where to adjust the knobs for the best temp... I had to fiddle with this for a bit and make some notes (which I can't currently find). Pre-heat your grill as you would normally - with all three burners on. After 20 or so minutes, pop the lid for a moment, drop 2 of the knobs to the lowest 'big' dot, and shut off one burner. Check your temp after about another 15-20 minutes - adjust accordingly.
My Weber is hooked up to the natural gas line. In theory, there should be no difference between the temps from propane to natural gas, but my internet readings tell me otherwise.
Important note: If you're picky about the doneness of your meat, use a probe thermometer with an external monitor - so you can watch the temperature without popping the lid. (This is also why using a side burner works well.)
I do not have a formula for the amount of time it takes to cook a specific size roast - I watch the temp and pull it when it's close, and let the carry-over cooking during the rest, bring it to it's final temp. If you're planning a dinner, with a specific sit down time, make sure you've got enough time in the front... let your roast wait for you... not you, for your roast. Beef absolutely needs to rest for a minimum of 20 minutes - longer if you have it.
Smoked King salmon was last weekend (so rich, so yummie) ... smoked cod is up in the next week or so.