I have a room mailbox in exchange online and I would like to publish its calendar online. Our default sharing policy is to only allow "Calendar free/busy" information. I see that I can easily change the sharing policy on user mailboxes, but is it possible on room/resource mailboxes?
It is, via the exact same parameter. Doing the actual "publish" part will be trickier, as the corresponding options will only be exposed when you are accessing the Room mailbox "directly". But you can always do it via the Set-MailboxCalendarFolder cmdlet.