Max simultaneous missions is actually 16, not 8, but yes, it's still good not to use those if you don't have to.
For your purposes, the crön will work a lot like a mission. You'll tell it to activate any time that you don't have the proper outfit, and set it to abort the missions when it stats. Make sure it has a %chance of 100 and a duration of 1.
I actually thought of another way of going about this that I'm pretty sure would work. It would be a less resource-heavy solution (since it doesn't take up missions or the active mission slot), but it's a little less intuitive for the player. What you would do is make the jammer outfit in two parts: the first part gives you the jamming/regen and energy drain, and the second part is the converse of that, making it a zero sum. Buying the first one grants you the second one, which should be invisible in the outfitter. Now here's the kicker: make the first one a fighter bay and the second one a fighter. The fighter should be of a ship type that is either invisible or tiny and a planet-type ship, so it can't be hit, and untargetable. Give it an escort type of freighter. That way, when you launch the fighter, your ship will lose the attributes it gives, meaning you get energy drain, shield recharge, etc. When you want it back, just tell freighters to return to the ship.
Like I said, it's not as intuitive, but it's easier on the resource load.