Seems like to do a good code review, the reviewer needs to know the scope of the code they're looking at, and where it fits in the big picture. Hell, maybe even the big picture too.

If you outsource code reviews, you lose that perspective and it comes down to "will it compile, and does it look vaguely correct?" which isn't super valuable.

That being said, if you can overcome that problem then the idea becomes a lot more enticing :)