I've experimented with adding a waist stay, and it does wonders for keeping the skirt up. Since the stay is several inches shorter than the circumference of the waist, I will need to attach the stay to the dress using thread chains or a casing instead of tacking it directly to the waist seam.
Boning would be a good idea, but it's too late for that at this point-- there is no way I am taking apart the bodice to add a corselette, for any number of reasons. In addition, I need the dress to be hand or machine washable for a number of reasons, and spiral steel boning would rust under these conditions.
Also, as Alessandra points out, boning would go against the flowy, drapy dresses of that time period, and I think it would prevent the bodice from draping as planned. I think the patterns' original plan for holding the skirt up was with a waist stay + belt, so I'll have to start looking into appropriate belts as well.
I'm thinking of adding another grosgrain ribbon stay at the top of the bodice, in addition to the elastic. I'll let you all know how that works out.