Actually, no such luck. A lot of fics came close. The closest being A Heart So Wild by Dark Dreamlover. Except, Buffy wasn't injected in that one and it was shorter than the one I do remember.
I just re-reviewed this entire thread. Yes, sounds confusing at first, but towards the end of the first page of this thread, I started remembering more. Maggie was vicious in this one. All she cared about was getting babies she could control and turn into soldiers. That's why most of the stories suggested sound like it, but aren't. I am beginning to think this was on the website BSV Diaries or something like that. I loved that site!
So sadly it wasn't found. Maybe someday someone can write a Dark Maggie kidnapping, injecting sperm of (preferably Spike's lol) into Buffy and putting it into their own story. This was such a bummer. I remember looking for this and coming up with a big fat nothing.