This is an interesting idea.
It would work kind of like an escrow, right? Where the "release/publish clause" could be tied to copyright expiration or other legal events like you say.
The problem I see is, how do you filter/reject the amount of stuff you would have to store without even being public? AFAIK, the IA has issues storing so much of the already-public data, so storing "dark" (non-public/unpublished) stuff would potentially mean lots of cruft and garbage so to speak.
However it does sound very interesting. I hope we can some day achieve truly permanent data storage systems were we could just dump all of this and not worry much about it again.
Edit: Thinking about it a bit more, how feasible does the following sound:
Anyone interested in helping the IA could buy a sort of Drobo/NAS that is able to store only IA stuff (ala Freenet). Everything is encrypted of course, and then only way to access the files is when the escrow trigger fires off, the private key is released at the IA archive and then every owner of the IA-box will have access to that particular part of the archive (as well as regular IA users through web).
It's kind of like an HDD preloaded full of torrents, and then the differences or new additions can be streamed to your local IA-box as needed. You could even filter what kind of stuff would you like to help the IA archive. For example, I'm a big fan of movies so I prioritize that category (up to a certain % so that no one category is forgotten).
Does anyone know if anything resembles this? I mean, I could very well leave a low-powered NAS to help the IA serve their content, store it for later use, etc. And I imagine (hope) that a lot of other people would too. It would be a way of donating electricity, space to a worthy cause.