I solved my problem, I am posting the solution in case anyone else has the same problem:
The Launchbox database files are xml-files, as anyone here knows. I simply imported them into excel, and used the following formula for my region field:
=IF(ISERROR(FIND("(USA)";A2)>0);IF(ISERROR(FIND("(Japan)";A2)>0);IF(ISERROR(FIND("(Europe)";A2)>0);"";"Europe");"Japan");"North America")
I exported the file again as a regular xml, and everything works perfectly. Generally spoken, Excel is great for organizing your database and doing bulk edits like these. Wonder which games have a missing manual? Just let Excel sort your games by the manual-field, and you immediately see everything you want to see. I wonder why noone recommends Excel for these kinds of problems (I've seen many threads like "How can I filter games which have "xyz" missing"). Brad, I hope this tip can help the community in the future (or is it it already a recommended tool for this kind of stuff?).
Best Regards and thanks for the input so far to both of you
Edit: I forgot to say what the formula does: The formula fills out the region field for "(USA)", "(Europe)" and "(Japan)". If there is no match, the field is left empty. Of course this naming scheme is for no-intro roms in this case.
Edit 2: Of course I mean the platform xml files!