Just been doing some more research and I've found this image from the Kyoto Museum...

This is dated c.1760's and it's a funny shaped corset for that period. I'd expect that kind of shape to be more late 70's and 80's but what interests me is the quilted petticoat.

They're not draped over huge panniers but it does appear to be the later styled 'french' side boxes that could be hinged up to help the lady through doorways or into carriages. 

I'm only having a good old guess but it's a valuable image.