An overhead thrust would be interesting. I can't imagine being able to do lateral and vertical swings very well when you are holding the weapon that way, though, so the bonus would just be range, perhaps? Or just that you can stab at a shorter range than with an underarm stab?
I haven't played the vikingr mod though, but I know what kind of stab you're referring to.
(Have yet to look at the video posted)
EDIT: I am watching the video and I wouldn't mind if the "aternate animation" was just an underarm poke similar to the poke the guy did in the video posted (at 1:45), just higher up. I dunno, it looks a little more like the one hander stab. The default underarm stab always seemed kinda funny to me...