I actually made a extension to the BigBlueButtonBN plugin so that the guest user can choose a name before entering.
You can find the Pull request over here: https://github.com/blindsidenetworks/moodle-mod_bigbluebuttonbn/pull/257
Unfortunately there was no answer from the developers yet.
Like the request and leave a comment maybe it will help so that it will be merged into the next release quicker.