Using Tag how do I cut and parse file paths into tags
run 'find' from a root directory searching for plain files (-type f?) and -exec a bash script on each file (or perhaps automator).

Looks like you'll need a bit of 'basename' and 'sed'.

Append to an empty string for each input path, and treat that as a list
for that file. I believe with 'tag' that you can pass a comma separated list, so you can append one (dealing with first having no comma) or post process using 'sed'. worst case loop over the list string and run 'tag'
each time.

'tag' might be smart enough to append tags it doesn't already have what's passed, otherwise the top of the script might want to bail if the file already contains tags.

Not trivial to write, but if you're not familiar with bash, tag and sed it
would be a good learning exercise.