Wow thats an interesting idea. That would totally suck to be on the recieving end of I would want it to be expensive as all hell.
The idea would be that it would come with Vet 1, meaning that you'd have to build some penals and keep them alive. But if you could do that you would have a niche unit that would combine well with either t3 (T34 rams) or t4 (by punishing tanks that pushed deep into your lines looking to snipe your SU-76's with a combo of AT nade/"penal main gun disable").
Number one reason I ever used ram before it was nerfed to hell was for destroying the maingun on an enemy tank.
Nothing else was more important than disabling the gun on german tanks. Immobilization or engine damage was secondary to eliminating the damage output.
Implementing an ability that disabled the main gun would necessitate a removal or rebalancing of abilities like button and ram.
Button <-> Ram <-> Disable main gun
Any two and you have ensured methods for disabling german armor. Honestly I kinda like that sort of reliability. However I personally feel that what would be MOST useful would be the ability to damage or destroy tanks ability to rotate their turrets.
Forget anything else, if turret rotation could get slowed or even stopped viz critical damage, then a whole new element of vehicle facing and positioning would be highlighted.
A panther with it's gun stuck to the side would have to make a choice between trying to fight more or facing the wrong side of its armor, etc. IMO that's a whole lot more interesting than trying to get point and click abilities to fire.
Button is a doctrinal ability meaning you have to pick a build with guards in it for it to work, and Ram is far from a reliable main gun disable.
As far as the disable gun ability, it would have a ridiculously small range, meaning that you would basically have to be sitting on top of tank IOT to do it, meaning that you would A) have to pull off a really good flank, or B) disable the engine first.
Now, this is also combined with the fact that Penal squads are extremely fragile, meaning if they have any sort of AI capability they are going to get chewed up and you're going to lose them, which you then don't have the ability to instantly replace that ability as it would come at Vet 1, meaning you need to build another 360 (320 with decrease) squad, get them vetted against presumably vet 2/3 Grens/Pgrens, before you could do it again.
[edit: you could make the ability work like a demo charge, with a faster set up time during which they take additional damage]
I just think it would give penals a bit more of a niche. There would then be a point in combining a penal squad in with your con squads knowing that if you could keep that penal squad alive it's going to pay off later when the powerful Ostheer armour rolls out.
Finally, this would give you an opportunity to nerf the Ram that everyone keeps complaining about slightly, or take away some of the randomness from it, while still maintaining the ability to disable German tanks offensive capabilities.