Ok, I have added a test case and it has actually allowed me to fix a small bug: I had tested the case where the parent of the submodule is a normal Python module and the case where it is a frozen module, but another small fix was needed to make it work if the parent is built-in itself.
Please be aware that I have only run the regression tests under Windows, as at the moment I don't have a suitable Unix box available.
Also, I have uploaded the test case file (test_submodule.py) separately, as I don't know how to convince SVN to put this file in the patch.
The new patch is against current trunk.
File Added: import.diff