Why not add SSRs such as:
- A weapon is never marked with a malfuncting MA, even if it rolls more or equal to its B# (or simpliest: A9.7 is NA)
- A unit possessing a malfunctioned weapon may not be overrun, nor can an ennemy AFV enter its hex (to avoid omniscience of the player who knows that the SW is malfonctioning when his AFV driver don't known it).
- Or find a procedure like the one about extra black chit for fing OBA upon concealed-only ennemy unit (C1.21)
But is it a SSR or are you trying to change the rules so as they fit to your vision of what a tactical level WWII wargame must be?
SSR are needed in a scenario to depict some effects particular to the situation (such as terrain changes, weather, the presence of an heroic character, AFV with no fuel etc...) or to avoid some gamey tactics that could "kill" the scenario balance when used (such as kindling is NA). The SSR you invoke may be useful in a scenario where the defense relies on key weapons (such as an AT gun), but i'm sure that there is better ways to balance the scenario (with some change in the OoB, to not rely on only one weapon).
To add a SSR because you "always hated when a tank can drive right up to a gun with no regards to life and try and destroy the gun, it was unrealistic." is not a good idea.
In this case, add a SSR such as:
-ASL rules are not in use. Play this scenario with a better game system.
Me, i'm often annoyed, when ennemy comes behind my broken unit to eliminate them in failure to rout. Should i ad a house rule allowing unit to attempt self rally at the beginning of the ennemy MPh?