Hi David, all,
thanks to everyone for the support.
The trouble was that the Python interpreter failed to find a library from which it was supposed to import some symbols when the Squeak VM was started from the Finder. While the Python VM binary could be resolved without problems via /usr/bin (Finder PATH) or /Library/Frameworks/Python.framework/Versions/Current/bin (Terminal PATH), resolving the library depended on the latter.
Putting the appropriate PATH value in ~./MacOSX/environment.plist and logging out and back in again solved the problem. :-)
Best,
Michael