Personally I would implement - or would purchase an account on - such a site where companies would advertise for investment, and would be required to provide a certain minimum of disclosure. The site would provide avenues for that disclosure (basically, a feed of reports issues by the companies themselves and perhaps also relevant news stories, a'la Google News) as well as investment portfolio tracking.
It would (still) be up to the user to verify the disclosure and make sure they are looking at companies that are disclosing the right quantity, quality, and purview of information - but the site would hopefully make it very clear what is and isn't being disclosed and how that compares to other companies.
Of course another key feature could be investor/analyst reviews, but this would need an extremely well-engineered system to prevent or discourage astroturfing & other social engineering schemes. Personally I doubt anything less than only allowing authenticated professional journalists (affiliated with reputable publications) could be acceptable, at which point you wonder about the real utility of such a thing.
I feel like this would make an excellent Startup, actually, and might do some work in that regard. I think step 1, though, is spending a lot of time reading about the ins and outs of the law. It would be very easy to grab a 'legal third rail' with both hands with this project and expose yourself to a lot of liability. I find that intimidating, but maybe not too much so.