Well, third-party products aren't made by Nintendo, so they're not guaranteed to work perfectly. And when the Kama came out, nobody knew about the MotionPlus, so it wasn't designed with that in mind.
I don't have the motion plus yet, I'm waiting for a Tiger 10/Sports Resort double buy on the 26th, so I don't exactly know where the plastic parts you're talking about, but have you thought of trying a dremel (sp?) tool to sand it down? Worth a try?
I played a few games with the Kama and enjoyed it. I stopped using it when I noticed it didn't register movements fast enough with certain hyperactive mini-games. These games include the Rayman series, WarioWare, and Sonic and Mario. I noticed any time I had to move the wiimote and the nunchuk really fast the kama would decrease my performance greatly. When I plugged in the wired nunchuk, I would immediately do better again. For games that don't involve nunchuk motion much, the kama is fine and so relaxing to use.