Just a technical note:
I doubt it would be that difficult for the cops to figure out how to find
www.txtmob.com. If you want to make it expensive for them to harass cyclists; instead of one flash mob you would be better off with smaller, multiple, simultaneous mobs at widely dispersed locations.
Another thing: If all the folks getting arrested are getting court dates, the traffic courts should be getting pretty clogged by now. If they are not getting court dates, then I would say that hundreds of folks being arrested without being charged with anything would pretty plainly prove harassment. I would think a lawyer who was also a cyclist might consider a class-action suit.
Dan
Dan