How about a secondary weapon that launches a little nano bot that attaches itself and starts sapping armor from any enemy it hits? The affected enemy mech can only remove them by going into repair mode (sounds simple enough right? but get hit with a few of these things and you'll be blaring for cover while they're still chomping on ya). Multiple hits attach more nano bots and increase the sapping rate. This way, with each hit, an attached tech's repair rate essentially gets lower and lower until they stop to repair. Also, by requiring repair mode to remove the nano bots, the affected mechs HAVE to duck out of combat.
I feel like this would make a good support fire weapon as it would both boost team damage output and reduce the effectiveness of tech pushes by forcing enemies to duck back into cover to repair.
Actually, it could probably be used in 1v1 as well now that I think about it... since you could essentially keep increasing the sapping rate while preventing the enemy from repairing.