I read a comment stating that there was a Dunmer Spetim empress - who was therefore dragonborn. Is this true?

We don't know if she entered into the Akatosh covenant or not, but yes there was a Dunmer empress who ruled over Tamriel for about 40 years or so before being assassinated. Funny thing is she was one of the most beloved rulers of Tamriel and her rule was one of long lasting peace, would be cool if she was a dragonborn though.

Septim is not synonymous with dragonborn.

What was her name?

Her full name is Katariah Ra'athim, which is a house that was absorbed into House Hlaalu, her family was in charge of ebonheart before the Dren's somehow got control of it.

IIRC, she was married to Pelagius Septim III.

Yes, that one. He ruled in name, but since he was quite insane, she took care of things.

She ruled after he died as well, meaning she would've had to enter into the Akatosh covenant in order to light the dragon fires which is the ceremony that inaugurates the emperor/empress. So, yea in a way she was a dragonborn just not a pure-blood septim which isn't required to be a dragonborn.