I disagree. It isn't the plugin's fault that Plex doesn't put the database in the application directory specified. The plugin can't really do anything that I consider a good solution to fix this. Plus, the OS partition is too small. This wouldn't be an issue if there was no data partition on the SD card (which is what OMV recommends).
I'm sure you are correct, by 'plugin' I actually meant Plex itself - it wouldn't be a problem if it was possible to choose the DB location from within Plex.