I posted a similar concept that I think is a bit more streamlined and reasonable over at Reddit this morning.

http://www.reddit.com/r/Bitcoin/comments/281ws5/pow_alternatives_to_avoid_51_concerns/

I don't want to hijack this thread, but I also don't want to clutter the forum with multiple discussions for the same ideas.  If a mod feels appropriate, please move this comment or let me know to start a separate thread.