What tool and technique do I use to achieve this finish in plaster?
I just got an answer by email from the owner of a company in France that specializes in artisanal plaster work. He recognizes it as a very thin lime-plaster that was applied with a bundle of leaves like these:



with a whipping motion. He calls this type of finish an enduit fouetté which translates as whipped lime-plaster. He suggests that we mix a very thin lime plaster and use a bundle made of olive leaves to achieve this finish. 

Here is a pretty good explanation of how these enduits fouettés are prepared and applied (including some videos). I don't see my exact pattern, but some of them come pretty close.