>After watching a Defcon talk about the idea that transponders self-indicate their position without signature

Older style (I forget the proper name) transponders don't indicate their position, only ID and altitude. Newer ADS-B transponders include GPS position.

It's an interesting point though. Assuming for a moment that a bunch of drone operators equipped their drones with ADS-B transponders, I speculate that there could be a chance of overwhelming an airport approach radar's ability to track targets. Could potentially overwhelm the electronics, or the operators.