The end result is he would still be able to vote with 5 million steem power just like he can now, it'd just be coming from many accounts.

I've been thinking about this for a while. I'd be interested in building a tool that detects collusive voting. You should be able to look at the posts that a account votes for and figure out which other accounts have commonly voted on those posts. If you have a 100% match, that's a good indication that those two account are owned by (or controlled by) the same person. My @ozymandias and @philipnbrown accounts are a good example: they vote on almost exactly the same set of posts.

Of course a clever account-splitter could try to outsmart this, but to do so they'd have to give up power: they would have to spread their votes out and not put all their voting power on every single post.

This fancy tool wouldn't be of any use if we didn't have a way to actually punish cheating (a la smooth or abit downvotes), and it also would probably get a few false positives.