I think that instead of a custom file format it would be better to just use something like sqlite (a well known SQL database format) or, since the db structure is so simple anyway (3 columns: path, score, last accessed), you could get away with just tsv, which is a text file that you can edit wherever you want.

This is also useful if e.g. you moved a whole tree of various directories to a different root path and want to keep all the scores.