It's a pretty thought...
I think it could work. I mean, dude. Think about it. Alright, so the monks do some scrying, or some other monk-y type deal, and found out there was this day that no one but Angel remembers. Turns out, on this day, Buffy and Angel were boinking like weasils. Turns out, on this day, Buffy was freakin' PREGNANT.
Alright, so the monks scry a little more, and they go into the future to find this daughter of Buffy and Angel's. Let's say, she's led a crappy life. Mom's dead, Dad's a recluse, whatever. The monks pluck this girl from an alternate reality and tweak her a little, insert new memories, blahblahblah. It's like she was recreated, because basically she was hollowed out to hold this energy and given new memories.
Or they could just use the basic template of this daughter of Buffy and Angel's, like her looks. Because, let's face it, Michelle does kinda look like a mix between David and Sarah. Anyway, as they create the Key, they take the template for this daughter and make it Dawn's form. Perhaps they even use her name.
It's possible. It just takes a little creative imagineering.