I posted a 5 player variant a couple months ago... scroll back a few pages in the forums and you'll see it. It requires the expansion, but you could probably do something similar with the base set.
The biggest problem with more players is that there are more attacks and so it becomes much harder to get a creature to survive in order to score. This slows down the game significantly.
I've seen several proposals to fix this but haven't been satisfied with any of them.
Thanks, I found your thread here! Seems to be a great idea to "only attack your neighbours".