I used to create maps for quake1 and quake2... And q2 supported breaking stuff.
However, it was mostly on windows etc. And the main problem came in with the "lighting" of the maps that used windows that could break. (A wall made from breakable material didn't make shadows)
The VIS algorithm or something didn't actually consider breakable parts as "walls". And if you used a breakable part to try to mend a "leak" it wouldn't work because of that.
An easy solution... but it will have a serious impact on the eye-candy of the game, is to replace all the "solid" walls with "glass" in the maps. These glass just need rediculously high resistance, so they don't just break for nothing.
Each breakable part has a "health" assigned to it, and this should just be high (20,000 for brick walls, 7,500 for wooden walls, and probably 150,000 for steel stuff).
(This is assuming a typical pistol does about 150 damage)
Penetration of walls should be done differently though.