I don't believe we'll be seeing a Mirror Excelsior.
The Mirror ships are different skinned and slightly altered BOFF layouts of free level 40 ships we have, as a means of letting players change it up, and also possibly get a decent ship without grinding dil.
Yeah I think all of this, probably just wishful thinking threads, do lose sight of the fact that we will be unlikely to see mirror versions of ships in boxesof ships that are sold in the Cstore.
On the other hand, there is a possibility we might see them in the Cstore.
It would certainly be an interesting idea, but I'm not sure it would make sense (yes, I even try to apply sense to the goofy MU).
The Terran Empire was destroyed after Spock had taken over and tried to create peace.
The current incarnation is a rather recent event and appearently their ships are based on the prime universe versions (like the Defiant).
Ships like the Miranda make sense since they are likely to have originally been constructed before the fall of the Empire (and might have been lying around in some depot), while the newer ships are based on the ones from the Prime Universe.
Ships like the Excelsior...well I'm not sure there was ever any interest in building them.
They were conceived in an era where there was no Empire after all and never built in the Mirror Universe of that era.
And the newly refounded one probably had its hands full building "new" ships instead of retrofits of older ships.
I'd sure love to see one though.
Second Terran Empire maybe? or.. you know... the not so cool "Terran Federation"