I've seen this as a payload for military UAVs (Hunter); it's a 700kg aircraft, though.
I don't think you could put the right electronics on a small quadcopter, but the gas powered quadcopters are perfectly capable -- anything which can carry a DSLR.
The nice thing about antennas is they're super directional (if desired), so really all you need is altitude, not necessarily maneuverability -- a tethered balloon or just being on top of a building would work fine. For SIGINT, you probably want longer loiter times than a small UAV will give you; a small UAV with wifi/cell would mainly be for tracking a moving target, or killing a target which happens to be radiating on a specific frequency (i.e. find/kill a certain cellphone).
In general the military cares more about cellular signals than wifi, at least so far.
Electrically powered multicopters can carry DSLRs quite easily. If it's supposed to carry expensive equipment, you'll want to use a hexa- or octocopter though.
Of course multicopter flight times, especially with payloads, are problematic.
I don't think a tethered balloon would be an ideal platform for such a directional antenna in any vaguely windy location, you need a lot of stability for it to be useful. I'm having difficult imagining any way to make that work which isn't essentially a stayed vertical support with the balloon ... probably not at net helping.
JLENS works with a variety of sensors (optical and RF) today, although I'm more familiar with the smaller systems (just a big tower with a payload on top)
Snowden documents (the ones Applebaum presented, possibly, if not others) confirm that US drones already do this in the countries our oligarchs don't like.
"In addition to the GILGAMESH system used by JSOC, the CIA uses a similar NSA platform known as SHENANIGANS. The operation – previously undisclosed – utilizes a pod on aircraft that vacuums up massive amounts of data from any wireless routers, computers, smart phones or other electronic devices that are within range." - http://privacysos.org/node/1323
I think it used a surplus army target drone, with an ardupilot flight controller, and either an n900 or openmoko cellphone as the base for the onboard computer and wifi, bluetooth and 3g radios.
The general idea was you could connected to the drone via 3g or wifi to control it, and use the other radio to intercept signals. If necessary you could offload heavyweight computation to servers on the ground.
interesting - seems like the reverse use-case would also sense. soldier with one of these uses it to block guidance and communication systems going into a drone. range would be an issue i suppose.
I think overall it probably depends on how one defines havoc, who is behind such and where it takes place (some village in pashtunistan or some university campus). After all, the same action being described in a another context could be called "peacekeeping" and be accepted as such by vast swaths of a population.
