Hi Jim
I imagine, as a partial solution to my previous point about temporary obstructions, the same technology that you are proposing for drones could be mandated for any temporary obstruction over a height of 100'. It could be argued that fitting such devices to temporary masts, mobile cranes etc. would actually be easier than trying to retrofit thousand of unregistered drones that will be in the air prior to any legislation being introduced.
Regards
TeeS