Reading David Revoy's post on trying to keep around high resolution PNG files has made me think again about the various difficulties in making a "complete and corresponding source" type free culture future possible. These seem like issues to me:

Free software's sources are usually comparatively slim as plaintext. Free culture's sources are usually large and heavy binaries. So, storage space, on your local computer and especially for long term archiving, seems to be an issue. We could say "just use archive.org!" though I'm not sure that's satisfactory.
Free culture does not usually have "build reproducibility", even for the assets that could be reproduced. Certain parts of the process could be makefile-reproducible, for example rendering XCF->PNG, or rendering a .blend file, etc. (To some extent this is accomplished in open movie projects like Tube, but not totally.) This is something that could be tackled, but is cultural.
Many sources are hand-altered after being rendered, ie "post processing". This cannot be mechanically reproduced. Our best bet here is to keep the original source around, and the file used to post-process around.
As for version control, it's possible, but nothing is ideal, to the point where @Bassam Kurdali told me that a number of frustrated artists have expressed their desire to create their own version control system just because of how underserved they feel by contemporary VCS'es. This is true especially for git, most of the command of which generally expected that you have the full history around (and even if you pulled down just the recent head, future commands may pull the rest down anyway). git-annex may help, but does not really provide generally a workflow for large files you wish to keep around. Subversion is still the most popular option for hosting large free culture project sources.
There is little to no culture of sharing source in non-software contexts. Who aside from the Blender Institute does it on anything major?

What can we do to improve on this?

(Even saying all this, I still have an angry task on my orgmode agenda to get the MediaGoblin campaign video sources uploaded to archive.org... blah!)