I agree that it doesn't sound very Star Trek-like for a Federation prison series. But what if it was a Dominion prisoner of war camp run by the Jem-Hadar like "In Inferno's Light"?
Yeah that could work as an adventure series. Or how about Tom Riker in the Cardie prison camp? We never did find out for sure that he was killed. When the Cardassian-Dominion alliance happened, things would have gotten a lot rougher for the prisoners - what did they do then?
Or how about a story of a character who manages somehow to exist among the Borg without truly being one of the Collective? Maybe from a resistant species, able to shield the Borg from such knowledge. If the Borg found out that his or her species was resistant, the Borg would try to exterminate them, which would give the character a motive to continue the charade.