The issue with shortcuts and starred items are that they are not true entries in Laserfiche but rather they act as database pointers.  The shortcut entry does not really exist other than in the database unlike other typical entries which contain content.  Since they don't contain content there is nowhere in the entry to reference an alternate repository.  Since they are simply pointers all context is limited to the database in which they reside ie. the repository database.  Each repository has a unique associated database with no real knowledge of other repositories.  I believe to allow for cross repository shortcuts or starred items a large rewrite of the underlying platform would be required.

There are some other options to accomplish similar results. 

• If your user base is using the windows client .LFE shortcuts can be created and stored in the repository.  These are actual entries that contain content and as such they reference the repository to which they are associated.  You could place an .LFE shortcut to an entry from RepoB into RepoA and it would launch the windows client and load RepoB's entry when clicked.
• One could use the SDK to create an entry in RepoA of type .URL which could open an entry in RepoB.  The user experience is not great however as the web client has not built in viewer for .URL files.   Essentially the user would click the .URL entry in RepoA which would download the .URL file, when they click that downloaded file it would open the RepoB entry in a browser.
• There is an existing  Workflow activity named replicate entries which allows one to copy entries between repositories though you would end up with multiple copies of the same entry in multiple repositories so that might not be ideal for your use case.
• If the shared content only needs to be available to users in a read-only capacity and its not private content that UserA is permitted to see but UserB is not, there might be a use case for using Weblink for the shared content.

I am sure there are other ideas but those are some options off the top of my head.