Hey Ed.
I checked around and it doesn’t look like dealers (or even manufacturers) list weight capacities in the specs; weird. Obviously you probably already did that. I checked H-D, Yamaha, Kawasaki and Honda. However, the following link is to an article entitled,
Getting Loaded: How Much Weight Can Your Motorcycle Handle? and states:
“
There is a simple way to tell the maximum weight that your bike is rated to carry by its manufacturer. Just subtract the wet weight (that is, the bike's weight with the tank full of fuel and the other fluids topped up) from the gross vehicle weight rating (GVWR). The GVWR is the maximum allowable total weight of motorcycle and its load, including riders, luggage and other debris.”
http://www.motorcyclecruiser.com/streetsurvival/overload/index.htmlSo, the dealers and manufacturers I checked don’t list GVWR either … lol. However, they list the wet and I easily found the GVWR for a couple bikes that I randomly picked simply by Googling the bike model and "GVWR". Examples I searched:
My XR1200: GVWR 1,000 – wet weight per mfg 580 = load 420 (I weight less than that

)
Kawasaki Connie (I've got to get one of these): GVWR 1,111 – 670 = load 441
Yamaha VStar250: GVWR 675 -324 = load 351
Honda Shadow 750RS: GVWR 957 – 507 = load 450
Seems to me that once you pick the bike, you can follow the article recommendation, but once you land on something, you could check with a dealer just for peace of mind.
Hope that helps!
[Edit: BMW lists the weight capacities for each model they sell.]