A high reputation on Stack Overflow does not mean that:
- The person will get the job done on time
- The person will work within your budget
- The person will be available for bug fixes and updates
Most freelance oriented web sites and services offer something that I think you'd find a bit more valuable, reviews by other members over time. While skill is critical to getting the job done, you probably also want someone who has a track record of shipping stuff of reasonable quality on time.
I would think that high reputation users on Stack Overflow that actively solicit freelance projects would at least mention their Stack Overflow participation in whatever profile space the service permits. Since, as others have said, there is no way to contact someone directly through Stack Overflow - I can't imagine why any service would prohibit such a mention.
I have, on several occasions thought of proposing a sort of marketplace, but always stopped short of suggesting it once I contemplated the massive administrative headache it would bring. Escrow disputes, buyer disputes, seller disputes .. meh! I'm not even touching on how excitable people become when money doesn't show up in their bank account quickly.
Beyond the careers site, I don't think anything else market related will surface as an official product. I could be wrong, though.