Well, this isn't
quite the same, but I was thinking of having a racial trait of "subterranian" (as seen in MOO2) that costs 2000 to 2500 points. You can't actually have more population, but you could simulate it by having "larger" facilities (reaching deep into the ground) that produce more resources/intel/research because they house more workers. These facilities would say produce 50% more than the typical facilities at each level. Thus you would have 50% more facilities (kind of).
I didn't actually execute this mod because there are just so many buildings that would need to be added to the game, and quite frankly, I like being married
But maybe some one else would like to run with it?
Cheers,