Got the seat out. Not too tough, really. Here's a few pics of the 17mm bolts holding it in:
These are the rear ones. There are two in the front in the same locations on the runners. There are also spacers between the brackets and the floor. I just left them sitting in their respective dents in the carpet, so I wouldn't get them mixed up.
Here's a pic of the other ends under the car:
The outboard ones have to be held with a wrench when you turn the bolts. The inboard ones, if they haven't been altered (like mine) are little blocks inside a tunnel, and don't need holding. My front inboard block had been replaced by a standard nut, and I think I'll be making the same swap at the rear, since I can't get the bolt to start in the threads.
This is what I found underneath had broken:
The black rubber "sling" that holds the cushion up is split all the way front to rear.
Here's my temporary fix:
I would say I "McGuyvered" it, but there was no duct tape used. I used 1/16 x 1-1/2 aluminum, cut to length, and bent at each end. I disconnected the clips holding the sling in place, then hooked the bent ends of the aluminum "slats" around the metal rods at each side of the sling assembly. Then I pulled hard to get the clips back into the holes on the seat frame. After putting the two cross-members in, I then put the fore-aft ones in just by putting them between the cross-members and cushion, and wedging one end on top of the rear of the seat frame. It's not perfect, by it feels pretty normal to sit in, and will hold until my new seat covers get here (I'm getting them from kmckmc's source), then I'll have my local re-trimmer fix the seat permanently when he's putting the new covers on.