This is already technically possible, albeit with a caveat. The bluetooth speaker won't be synchronized with the other zones.
1. Pair your bluetooth speaker(s) to a Mac, Windows, or Linux machine
2. Go to the server interface at config.plethra.com, where it should show up as an output device (it may also show up as an input device if it has a built-in microphone)
3. Hit the green + icon on the output device (you may also want to rename the speaker and give it a different icon)
Adding support for synchronization and auto-discovery of Bluetooth devices would be an excellent feature request. I'd suggest you add it to the list here.