Since i've never used double blind rules, but have read them here, many a time, that to me seems a simple solution.. Though for the 'visual' spotting, How close does one have to be?
I haven't actually used them, either, mostly because it would require 2 more people to do it, and my play time is already dedicated and tight.
But from my quick perusal, visual spotting is dependent on the environment and unit type. Infantry in Pitch Black having the hardest time seeing anything, with Aerospace at Altitude in Daylight (no other conditions) having the easiest.
One fun set of thoughts for C3 & ECM in double-blind:
1) If your unit has ECM on, it is revealed to all parties
2) If your unit's C3 is on, it is revealed to all parties
3) if your unit fires weapons, it is revealed to all parties
So you will have both sides carefully deciding if they want to have their ECM or C3 on, and knowing that their opponent is making the same set of choices. If you see an opponent and thy can see you, then you might as well turn on your ECM and C3. If you are in cover you might turn on your ECM to help an ally that is nearby, but you your opponent now knows where you are and will be moving units to shoot at you. Unless you are using that as part of your plan by mounting the ECM on a fast light unit and will be getting it out of range?
Similarly you can have one unit opening fire because they snuck up behind an opponent, but their weapons fire means everyone now knows what hex the shooter is in.
An interesting idea, however, a little off topic and I rather disagree. Sensors need set to look for C3 in order to notice it.
ECM would be noticeable, but it wouldn't pinpoint your exact location. That would defeat the point of ECM.
Weapon's fire would attract audibly, and MIGHT attract visibly. Not all weapons will attract sensors, though. Radar doesn't notice AC or Gauss rounds very effectively. PPC fire, though... Maybe? However it would have to be calibrated so it wouldn't get confused by a thunderstorm and generate false hits.