With OSX 10.12.3 and Picard v1.3.2, it was possible to open a folder via applescript. eg:
tell application "MusicBrainz Picard" try open pathToFiles end try activate end tell
In v1.4, this no longer works. Picard itself still opens, but no files are added. I’ve also tried this with the dev version mentioned here that fixes the drag & drop problem with 1.4.