actually you can. On the 002's, they actually screw in to the rim even though they are truly one piece. The 501's are just glued on there. Please let me know if you find one as well since I lost one of mine on one of the rims.
yeah, rather than casting the rivet looking circles into the wheel, sportmax actually inserts the pieces after casting, so there's actually a rivet in there which makes it look much closer to a multi-piece wheel. I've never heard of a rivet comming loose or falling out, although i suppose it's possible with that type of design.
Sportmax warranties all of their wheels from any sort of cosmetic defect, and I have no doubt that would qualify - definatley get in touch with the manufacturer and get an answer straight from them - they include warranty info/contact info in a pamphlet included with every wheel.