How about something along the lines of Blackberry's Balance feature?
Connecting another half, provides access to a secure/encrypted "other half" of the OS (or at least a sandboxed environment)

or a developer's other half, which sets up preconfigured tools and a sandbox for testing.

This would be a great way for Jolla to sell the product for normal consumers and development too, as it could remove some advanced settings components in the UI that may be inadvertently (or stupidly) activated by users, creating issues.
If you want to develop ON the device, you can opt for the "Developer" half.