Adding a command-line option to specify (override) the directory to use containing INI file, plugins, etc.will allow use of new (beta) version without affecting use of current (stable release) version.  For example (on Windows):

picard.exe -d --config=C:\Users\bob\AppData\Roaming\MusicBrainz_v2_beta

This would use the INI file and plugins found in the_* MusicBrainz_v2_beta*_ directory rather than the default MusicBrainz directory. This way the user could run multiple versions without having to rename directories before starting each version.