I am considering using the pattern of an open gown c. 1795 from the Victoria and Albert Museum that Norah Waugh published in her book, The Cut of Women's Clothes and the drawstring gown from Jennie Chancey's collection.
The Gallery of Fashion at http://locutus.ucr.edu/~cathy/heid/heid.html) shows the open robe worn as an outdoor garment. "Robe a la Turque" [below] is from November 1794 and is a nice example of an afternoon dress.Empress Josephine [below] wore a sleeveless open robe at her coronation in 1806.
1808 Princess Borghese
"Dress (open robe) [English] (C.I.37.46.1)". In Timeline of Art History. New York: The Metropolitan Museum of Art, 2000–. http://www.metmuseum.org/toah/hd/dreh/ho_C.I.37.46.1.htm (October 2006)