Using a global.db file would probably lead to too much wearing on the SDcard. I wanted to copy the global.db file from each directory to the root so that UDS (the file viewer) could add/update thumbnail/title/author. (See also other thread.)
So next steps will be to see if writing to global.db can be skipped (this will result in more code changes).
The alternative proposal of Iņigo to write a complete new file manager is also a promising solution.