This is actually fairly simple to mod in. (And thank Alex for the improvements that made it possible.)
Even supporting specific mirroring for specific mounts is pretty doable; my mod doesn't do it, but since mount point ids are strings, it wouldn't be hard to set up some mod's ships with something where, say, any mount ending in "_m" will mirror any mirror-capable weaponry.
That even plays into the reasons I made my mod as a mod + utility library. If mirroring based on mount id is something some modder would actually use, I can roll that into the library portion.
Edit: Of course, that it's easy to mod in is, perhaps, slightly orthogonal to the original request for supporting such a feature in the base game. Which would be neat if it happened! But until then, at least the modding side of things is doable.
