There is currently no option to do this. This would also be pretty difficult to implement as it would require completely changing how the current commenting system works.
Really? I've thought about this too, as it would be very useful. I don't know much about the MW code, so I thought it should be as easy as changing $pageContents = $pageContents + $newComment; to $pageContents = $newComment + $pageContents; (that's pseudo-code, obviously)
Guess I was a bit naive then...
-- F.