In the short term, one thing we could do is simply make changes to the System package and other core packages directly in the 39a repository, since we don't have master repositories for these packages yet. I hesitate to suggest this, though, as this is still too centralized a model. But if we have trouble finding package maintainers, we may end up having to do this for some packages in the short term.
My impression is that we should go slowly and do not go crazy and decentralized too fast. For well identified packages such as MC this is really ok but for the others this can be really a mess as I mentioned.
It is still true that an outsider submitting a bug fix should submit a changeset via Mantis or whatever current process. The package maintainers would be checking Mantis periodically for fixes from the community, and incorporating them into their package.