[npostavs]

A text file with a list that maps the IMSLP numbers / file names to the tags?

Yes, that's what I'm generating currently. For the batch import, I translate to a series of mkdir and mklink commands that produces the needed file/path names for tagging.

Did you try CSV import? Currently CSV import is limited to one CSV file per PDF, so you need to organise something like a batch import of CSV files

How would I organize a batch import of CSV files? I thought CSV is limited to one PDF, so it's useless for me.