Can I get mp3tag to remind me I've forgotten to fill out the composer field and the file isn't quite ready to be moved yet?
How do you move files? With Mp3tag's Converter?
If so, build the following function around your currently used formatstring:
This will leave files without a composer tag field where they are.
You know that you can display the composer field in the tag panel? Tools>Options>Tag Panel
Just to get a "reminder" it would suffice to apply a filter
%composer% IS ""
THis works even with "invisible" fields. (When they are invisible, it is rather hard to edit them without opening the extended tags dialogue, though.)