The Mirror Universe guys in game use one of the Jupiter Uniforms colored grey instead of black but with the traditional Blue/Red/Gold for departments.
Obviously those aren't available in the Foundry so when I made my Mirror Universe mission "Over There" I put NPCs in Sierra/Antares uniforms or the vice admiral and veterans jackets colored the same grey. I also used the mirror sash.
Check out the latest episode of The Foundry Roundtable at StarbaseUGC
Found this on the STO wiki.
I'd use a darker version of the Sierra 1 that I normally use, maybe more skirts and open uniforms for females, Sash that will get replaced with Ent era dagger belt... now if only the TOS Terran Empire badge wasn't reversed on everything but TOS mirror uniforms...