There are lots of canned plasters out there, but I doubt that any of them will be a stop/start finish. In fact it would be much more difficult and you will still have to go up the wall.

I think you should stick to what you know best, especially when the project is not the easy one! Dealing with the lagrge serface and have an unfamiliar material to work with???

Here is a tip for trade off work complications. When I give my client a deal, I expect her/him to realise that. (I assume that your dentist knows that you are doing him/her a favor in a way.)
I would give my regular price first, than apply a discount to that price. Now we have a "special person" price. Than I explain that I can trade of my time and skill but since that would be a "now money job", I let person know that I cannot pay for special setups and assistance from my pocket. In your case you need skafolds and helper (to hand you paints and stuff). I would pass this expence to the client and vola! job well done! If the client can not understand that than you better stay away from dealing with this person.

Anyway this is just my opinion.

Ilia Anossov
http://www.truefresco.com/anossov
http://www.truefresco.com/workshop