Currently , without collaborative repository, the add-value for webmasters where is the add-value?
The better solution will be to have this service linked to the public svn.
Probably an idea which need to be managed at a core level ...
One open alternative will be to offer to developpers the possibility to add directly in their module/plugin/module update link in the common xml, to offer a "by default" link to their prefered repository.
All my best for this projet!