eh, that just seems implausible. and without the lack of resources from the other nations, no race would be able to develop technology needed to attain space flight.
Um... there was a Space Station in the 3rd era, until a battlemage blew it up trying to escape.
Besides, the OP already said he was thinking of breaking the provinces into separate planets.
Also, what province-planet couldn't attain Space Flight, which is just a variant of Conjuration magic?
In Daggerfall, the main character went on a Spacewalk.
Heck... you even go exploring distant planets in Oblivion: At least three of them, actually!
Mehrunes Dagon's planet, Sheogorath's planet (Which you become God-Emperor of), and part of Vaermina's planet.
You may have also visited Boethia and Peryite's planets, but they looked like just some colonies on Mehrunes Dagon's planet.