Another option is to escape '/' in tags when turning them into filenames.

Let's somewhat arbitrarily say we use ',' as the escape character.

',' in a tag would be replaced with the two-character sequence ',0'
in the filename.

'/' in a tag would be replaced with ',1' in the filename.

(And so on for any other characters we need to escape.)

This would give the user the behavior we want, which is to be free to use
'/' in tags.

The implementation detail of escaped tags would be hidden from the user
unless they dug into the on-disk format by hand.