[Suicide Junkie]

Combat only starts if a non-cloaked object can see an enemy.
But if you weren't planning on having minefields be stealthy, that would work.

Just launching zero-move drones (with afterburner ability for combat move) into a sector would work fairly well.
Having the monthly supply use until they fizzle would be quite effective at preventing an unlimited number of them from being stashed in a sector.

It will still result in piles instead of system-spanning fields, but that's not too bad.

You should probably include the option of cloaked dronemines; they would be useful around planets and at defense bases...
- Once combat starts with the enemy attacking your planet or base, the dronemines would activate and go forth to kill.