I just had an idea - if I were to use the auto-imort feature to import a chunk of the 100,000 files each time I run MC15, that could eventually get all my files loaded, although none of the ratings would come across.
Could I then export an xml playlist of all the rated files (about 10,000) from itunes and import that playlist into MC15? Would that give me duplicate files, or just copy across the ratings?