A modification of the copyright system that satisfies both the need to keep the public space open and the rights of ownership for intellectual property would be extending fair use to fair recovery. If copyrighted material languishes in the private space of the copyright claimant and a case can be made that the property rights of the material are not being exercised fairly (i.e. at non-ridiculous prices) then the rights to fair use should also include the right to obtain the material in the first place.

Lessig points to a suggestion for a copyright extension tax which adds a small charge to copyright extensions. Failure to pay for the extension immediately makes the copyrighted material available to the public.
This accomplishes approximately the same thing, but not quite. First of all people might object to the tax as 'yet another way to put money in government pockets', and secondly - it does not prevent the censorship by ownership that is becoming commonplace. To me that is the real target for the activity to keep the public domain open.

You might argue that the cost of litigating to exercise the right to fair recovery would likely incur a higher cost than the copyrigh extension tax, but that goes both ways and it is entirely likely that it is cheaper for copyright holders to make their material available through some kind of library system if not to the general public. Many countries have rules like this for archival purposes anyway.