I've been thinking a lot about this kind of thing recently - and put a prototype up of htvend [1] that allows you to archive out dependencies during an image build. The idea being that if you have a mix of private/public dependencies that the upstream dependencies can be saved off locally as blobs allowing your build process to be able to be re-run in the future, even if the upstream assets become unavailable (as appears to be the case here).
[1] https://github.com/continusec/htvend