Dan, the clutch on the Harley works backward from what you experience with brakes. With brakes, as the pads wear the pistons are pushed out further and more fluid moves from the reservoir into the caliper. If you add fluid, when you finally replace the pads and push the pistons back into the caliper bores, you have to remove the excess fluid from the reservoir. With the Harley clutch, however, as the clutch discs wear the pressure plate moves further into the clutch forcing the release rod back into the actuator which in turn forces fluid back to the master cylinder reservoir. As the clutch discs wear, the fluid level in the reservoir actually increases. There is a warning in the shop manual about overfilling just for that reason. I tend to wonder just how many of the dealership folks doing service intervals understand this, and just how many hydraulic clutches leave the factory or the dealership with too much fluid.
A defective master cylinder that doesn't allow fluid pressure to be relieved could also cause a slipping problem. As you noted, if the MC piston doesn't fully retract it can block the port and keep the system pressurized. With the recent changes to the master cylinders (cheapening), I wouldn't be surprised if there aren't more than a few defective ones floating around. This isn't a widespread issue with earlier CVO's, so I would tend to suspect recent design and quality changes as a root cause. It also isn't being widely reported by owners of other '09 and '10 models with the cats in the collector, so I don't believe it's just a high fluid temp issue either. High fluid temps might exacerbate the condition, but I don't really believe they are the root cause.
Jerry