Everything you want to do can be accomplished with Unity except requiring a PIN. I can't think of a way to accomplish this with Unity alone. If you happen to be running IPCC, this is fairly simple with a script.
You would need to create an entry call handler and one call handler for each MeetMe Conference number. Each MeetMe number call handler should have a blank greeting and be set to attempt transfer for itself as the after greeting action. Also, on the Call Transfer page of each call handler, set the Announce option.
Hope this helps.
Brandon