Gisella,
This is a really interesting concept, but even the smallest (full term) newborns' heads (less than 5%) are about 13 inches in circumference. That's a lot bigger than 8 inches. So it's really not the same. I saw my best friend give birth at 6 months to a 2.5 pound baby, and his head was about 11-12 inches in circumference or so. I don't want to say he just slid out no problem, but she didn't have to push at all and there was no tearing or anything. She recovered in just a few hours.
I guess my point is, 8 inches seems rather large, but it's not physically impossible. I'd try it definitely, maybe with a lot of lube the first few times.
I'm still not using my own sign in.
Snoozan as JBT