The Robe a la Turque is coming along nicely! I've got the pink undergown nearly finished. I made it a round gown over both panniers and a bum roll. In looking at the pictures it just seemed that the skirts were much fuller and wider than a bum roll on it's own could acheive - and even with both it is still far narrower than this!
I need to sew on the hooks and bars up the front, add the lace ruffle around the neckline and finish attaching one of the sleeve cuffs and it's done! I made the cuffs and the ruffles easily removable so that I can wear it as just a plain pink round gown if I want to. As for the overdress, while my intention was to have it's skirts come forward a bit more than this plate, it's going to end up looking quite similar to this when it's finished.
Once I finish up the pink gown and wrastle my stays back on the dress form again I'll post some pictures.