Well I just had my cleaning last month but have had multiple cleanings because I have rsd for 4 years. I am also under orthodontic care. This time though after my cleaning I had a cavity so had to get a filling which made me anxious. I am not nervous of the dentist but of the rsd. I informed my dentist I have rsd. Not only did I inform I explained the condition how though it is not in my mouth it can spread there and extra care needs to be taken. Some doctors or dentists say they know about rsd but they don't so important to tell them. It is good though this was refered by your pain doc but I still would tell the dentist. I am ok in the mouth area since having it.
FOr you though I cna understand since rsd is in your mouth. I would call your pain doctor and talk to him pre dentist apt.
In terms of meds for you I am not sure. Could you also bring some ear phones for music? Maybe they can give you laughing gas? I know those don't keep the rsd at bay but I am talking about for the inner anxiety.
Hang in there.