After some hours of additional trials I have found a workaround that actually seems to work all the way.
1. A model branch of package A is initially exported from model A. The model is version controlled and stored in Subversion under trunk.
2. A formal release of A is created by copying a selected set of XMI files and the EAB-file to the subversion release directory (using Subversion, not EA).
3. The EAB file and all the XMI files are subsequently edited (in a text editor) to reflect their new location within subversion (Search & Replace).
4. The formal release of package A is imported as a model branch into a new model. The model will then contain version controlled packages that are stored in the subversion release directory. People working in model B can now work with the formal (read-only) release of package A, while other people are continuing their work on package A in model A.
5. An updated release of package A (from model A) is subsequently made available by repeating steps 2 and 3.
6. People working with model B would now want to right-click on the package A and choose "Import as Model Branch..." again, but this is not possible. Instead, the following procedure seems to work:
6a. Temporarily turn off Version Control on the top most package. This can be done by right clicking on the package and choose "Package Contol"->"Configure..." from the context menu. Un-tick the Control Package tick box.
6b. Right click on the same package again and choose "Package Contol"->"Get Package...". Choose to get the updated version of the correct package XMI-file.
6c. Finally, right click on the same package again and choose "Package Contol"->"Get All Latest" to update the child packages as well.
The connections are preserved!

I think that this shows that EA is actually capable of version control branching. However, Sparks need to make some minor modification to facilitate this procedure even further. Suggestions:
1. EA need to implement something similar to the command "svn relocate" to replace step 3.
2. Steps 6a, 6b and 6c, should not be required. Instead a warning could be displayed after choosing "Import as Model Branch" where the user must confirm that a version controlled package is to be replaced by a new one. Minor change.
/Hans