First, let me say that the Duke, Duke R, Hypers, Tounos all have their fine attributes and a capable rider will be doing most if not all of the typical "supermoto" manuevers within reason and capability. (notice I didn't say safety, Where's that in the SuMo handbook??? )
Supermoto by definition is a melding of the moto and roadrace track. therefore the common attributes between the two would be: Wide, grippy rubber, high power to weight ratios, and specialized suspension. Seeing as now the slant is more toward the moto aspects with current track configs I would say that the converted dirtbike would be the true supermoto, that is, until the peramiters of the sport change as it evolves.
So, going on that logic, I'd have to say much anything over 300lbs would move a bike out of the "true" specialized motard category and into something more ubiquitous such as the "naked sport" or "dual purpose".
WANTED: ONE GOOD WOMAN... Must have motorcycle license, own tools, proficient in bed/kitchen and have thin figure in order to be able to walk between the numerous bikes, quads, trucks, tools and extrenious crap between the garage door and the house.