All I can say is that Diablo SSU update files are "in the wild". I just updated my N800 from earlier Diablo to the latest Diablo yesterday, at home, using SSU...

Also the System Software repository can be edited: I did that before... when trying the Chinook betas. Somewhere in the system is a text file that contains all the repository information. There is one line that makes any particular repository "locked" or not. Just editing that text file, and then saving, you can edit the System Software repository.

Sorry, do not remember where the file is... it is so long time I did work with that.