Look, I definitely do not support required plates or even undisplayed registration for bikes (much less peds). But if I could donate 35 bucks (car registration IIRC) to the city or state and get a little plaque that says "I support the roads of XXX county" or whatever I think I would get one. Not because I think bikes should need them, but because I'm sick of the b*****ing and just want to stop it.
As for enforcibility (sp?) I see countless pickup trucks with unreadable plates, no one clears the snow off their plates in the winter (didn't their dads make them do this like mine did?), and there are a couple of cars on my route that have had the "temporary" plates taped to the rear window for over a year now. Good luck doing any better with bikes.