I brought this topic into the last Community Meeting. As expected, Jolla can't really comment on these speculations, since they are only speculations (what can they really say about it?). But James pointed out that the backports (improvements to SFOS components) from their licensing partners usually affect packages from the MER project, so they are already open source and can already be tracked and reviewed.

Regarding publishing the code of the yet closed parts of SFOS, Jolla's argument seems to be that they won't be able to handle additional stuff like reviewing commits and pull requests from the community. They also seem to be afraid of forks being made when they release the code.

You can read all in the Log of the meeting here, starting at around 08:08.
http://merproject.org/meetings/mer-m...08.00.log.html