"This would allow a filing, probably on par with a preliminary patent filing, which would establish a pre-existing prior art."
Such a system already exists, except that there is a nominal fee. It's called Research Disclosures and USPTO examiners are required to search RD publications for prior art. Added advantage is that it allows anonymous publications, for businesses that don't want to disclose to their competitors what they are working on.
Making it zero-cost for the submitter would probably lead to spam, and anyway there is some work/cost involved (writing it with broad claims, making it searchable, storing it indefinitely). For zero cost prior art establishing, you can publish it on a mailing list with publicly accessible archives. A problem is that it's less likely that the examiner finds it there. As we see with Samsung/Apple, invalidating a patent with prior art once the patent is assigned is much more diffucult.