[sciurius]

Based on my own situation and experience, I took a different route which may contain aspects worth to consider for MSPro. It goes far beyond guessing metadata based on file names.

I also maintain all my music on an external (Linux) system, and rsync it to the tablet. Then I use MSPro to do the batch import. So far nothing fancy.
On the pc I already had several databases that contain song information, including metadata. So I wrote a tool that extracts the metadata from the databases and stores it directly into the MSPro database. This works fine for me, I can quickly add all the relevant metadata (song title, artist, composer, key, tempo, signature, capo, ...) to the MSPro songs. It is, howver, pretty much hard wired for my situation.
I can imagine that it would be possible to supply externally generated metadata information about songs to MSPro, for example in the form of a JSON data file.