I find panniers rather graceful to move around in -- like they force you to be graceful bec. you think about where you are in relation to things & how you move. Usually, I'm rather a klutz, but I move more deliberately in panniers, so I think it looks prettier (same w/hoops & bustles, they all slow me down! I totally run into walls in modern clothes ;-). And I can't think of a sacque-back gown w/out panniers off hand... if so, it wouldn't be common, afaik.
I've found that my JP Ryan panniers collapse up pretty easily (perhaps *too* easily; she suggested adding the eqivalent of a "musket-shot" (was it a musket or a mini-cannonball? I know nothing about Rev. War ordnance, obviously) to the outer bottom edge inside.).
The pattern was super-easy; if you can get/cut properly-size hoop wire you should be able to make them up in an afternoon.
And they make your waist look tiny, proportionally speaking!
Hmmmm....I can't think of any image with a sacque-back without side hoops of some kind. If you don't make them really wide, they aren't hard to dance in -- I find they actually help with the dancing because they keep the petticoats more away from my feet and from between my legs. Driving is a bit of an issue. When I had my grandmother-mobile, driving while wearing side hoops was only mildly annoying. Now that I have a Yaris with smaller bucket seats, the hoops and I don't both fit very well in the seat. I can squish in for a short ride, but to drive the 30 minutes home from a dance program I usually take off my hoops so I can fit in the seat and my gown so that I can reach the steering wheel (the narrow cut of the back holds my shoulders too far away from the wheel).
I have only moved around in pocket-hoops, but in those I have never had any problems with moving or sitting, except when it's been crowded. Panniers are larger so I guess they could pose a problem, but pocket-hoops can be sort of folded upwards when you sit down. :-)
Having just returned from SalonCon, where a few folks had steampunky outfits, I've been contemplating what mechanical devises and accoutrements I can add to a Victorian outfit. This of course makes me contemplate twisting OTHER historical periods as well. I think all your commenters are agreed that some sort of hip-enhancing undergarment is required, but many of us empathize with the dificulties in accomodating historical "largesse" into modern proporations for travel and movement.
Perhaps you need a nice little pulley-and-chain mechanical contraption to "roll up" you pocket hoops, as it were. A nice decorative chain on the center-side "line," passing through a polished guard riveted on to each successive hoop, and drawing up at the side waist onto a decorative chain reel? A series of umbrella-like hinged wires? Or to go modern, a shock-cord-and-fiberglass-tube arrangment like a tent-stay?