For any Mythbuntu specific components, anybody in the Mythbuntu team (http://launchpad.net/~mythbuntu) will be able to modify the items and request someone with upload rights to sponsor to the archive. 

If you aren't a member of the Mythbuntu team, you can still push to a personal branch and request that your changes be "merged" into the appropriate Mythbuntu branch.

All components of the Mythbuntu package set are in Universe or Multiverse, so they can be uploaded to the Ubuntu archive to be built by:
  • An Ubuntu core Developer¬†
  • An Ubuntu MOTU
  • A Mythbuntu developer in the mythbuntu-dev team
See the developer policy for more information about joining the mythbuntu-dev team.