Plating bikes serves two possible purposes, one is a tax grab, the other is to reduce theft, and reducing theft only works if you could actually absolutely identify all bicycles. I can't see that working well, especially for older bicycles, look at the number of threads that start: I have this frame, and I have no idea what it is, because someone scraped off the decals and dremelled off the stops and hangers then painted it black and the only number on it looks like it could be B,E,F,H,I or T then C,O,Q or 0 then a number that looks like 23 and there could be 2 or 3 numbers and maybe another letter. 99% of the time, it's simply a tax grab.

Protests usually come from 3 sources, one is residents who have bicycles, sometimes several who will be majorly hit by the new tax, second is environmental groups who will see people deciding that the extra tax is too big an extra burden and just continue driving, third tourism boards, who see people who would bring and use bikes into the states, avoiding that state because of the extra tax.

It's been done before and usually the cost to run the program costs more then the fees generated, so they quit after a while.