Interestingly I've wanted to do this for years. The first time it occurred to me was when BrickShelf started going dodgy. I though... "Darn! All these MOCs that are in one seachable place! How can we replace this?". Around that time I was a bit doubtful of the future of free hosting due to several sites dying on me so to me the solution was for people to self-host and then all we'd need was an index... People could then post MOCs on this magical index, including where they posted it on sites like Flickr and Brickshelf so there was a degree of reliability and then people could vote on MOCs... blah, blah, blah.
But I've never got around to actually doing it (SiegDex is supposed to eventually cover that for EB at least... supposedly ) due to other EB commitments and that darn real-life thing. I don't rule out it happening, but with the way my life has been going in the last few years it's probably not going to happen.
But as Flipz said ReBrick is very similar in concept. I'm still not sure if I like it or not, but for the short term at least it's the closest there is to what you want!