Figured I'd better deal with this now and not at the last minute. The white gown does not work over the stays. Both are from the Sense & Sensibility pattern as printed and published by Simplicity, though I did use the S&S directions from the original version of the pattern. These stays work fine with my green gown, which has a higher, gathered neckline. However, with this gown, the stays create a ridge all across the front of the bodice, right at the mid-way from the neckline to the high waistline.
I took a picture, but it's not quite as obvious here as it would be if the gown were actually buttoned. Unfortunately, I cannot button this one myself, but I think you can get the idea anyway.
I'm thinking of making up a second set of stays, using the gore pieces for one cup-size smaller. This would pull the top edge of the stays in towards the body, making it so that the edge would push less against the front of the bodice. Do I need the smaller cup-size? I'm not sure, but the experiment can't hurt anything.
I've also considered pulling the pattern for the bodiced petticoat, from the La Mode Bagatelle pattern for this period. I haven't done so yet, so don't know if it can be worn INSTEAD of stays, or needs to be worn with them. If it could replace stays, I think from looking at the sketches that it would fix this problem.
Any thoughts on this would be most welcome!