If shooting loch put the gun on mallet-length (or longer) repair cooldown, would it be decent? TBH I'm a little surprised this problem wasn't forseen by our mighty Hand of Balance, but whatever. Roll with the punches.
This isn't the first time I've seen that suggestion, so it's something I've considered. Dunno if they saw it here and mentioned it in a random lobby, but I've definitely seen it discussed before. However, my response to it is two-fold and I'll explain each aspect a bit.
Firstly, if it were implemented I believe that the cooldown timer would be contingent on factors like clip size, rate of fire, reload time, etc. A straight timer would be more beneficial to guns that could sustain fire for a longer period of time without worrying about the timer at all (gats fit here pretty well), and get their job done before the gun breaks. It leaves them more vulnerable to breaking with incoming fire, but the same applies to guns the way they operate with loch now.
Secondly, it feels entirely arbitrary. It's the introduction of a new mechanic or rule that effects only one ammunition type and thusly becomes difficult to gauge in terms of balance. The same could be said to old lochnagar and the damage dealt to the gun (breaking lights entirely, but it was only really used on a Phobos in the light gun category), but even that paired with gun reload speeds, the repair ability of the pipe wrench, and common engineer positioning meant that it was almost irrelevant as a reason to not use lochnagar.
I don't know the game prior to lochnagar's introdcution (or if there even was a time before lochnagar's introduction; I'm ignorant here), so I'm looking at it purely from the perspective of mechanics that already existed when I began playing. I think when you start bringing in special rules that revolve around one particular thing you're having difficulty balancing, it means that it doesn't fit with the rest of the game and might as well be removed.