In a webcast, too? I don't think that is possible. At any rate, we don't want this staff person to be able to unmute. We want only 3 specific people to be able to talk. Some less tech-savvy people mute and unmute at odd times.
See LogMeIn Help info below:
How do I promote attendees to co-organizer, panelist or presenter?
It is not possible to promote attendees during a webcast webinar. If you would like an attendee to become a staff member, then you must add them from the Event Details page. Then they will need to exit the session and rejoin using the special co-organizer or panelist Join URL that you provide them.
Thanks for confirming things @Golder .
Your panelists should not be able to unmute themselves if the event organizer mutes everybody after the broadcast begins.