redssrloaded: "What is a rumble seat...?"
You need to watch a few more old movies

Rumble seats were options on both old convertables and hardtops. A late night ride by the second couple, in the rumble, usually involved blankets and much
snuggling.
Troy: "vendor to produce them because of legal concerns"
Can't be any harder to mount properly than those removable bench seats you see in a lot of minivans. Be a snap for gm, but their overhead is to high for something like this. No this would be a job for sombody like the crew that puts the ragtops on the Mustang. (That's done post assembly line, last I heard.)
Peaches: "just how do you get in and out of it without stepping on a body part...?"
The old cars had step pads, permemant or retractable, in strategic locations. Plus, it was not considered a bad thing to just step into the middle of the seat, on the way in.
While I'd say that it should be removable, I'd have to admit that "removable" can be stretched to cover a lot. The cargo cover is a case in point. My dad's SnugTop is easily removed by two people with no tools, while the SSR's seems to be more like trying to remove the trunk lid of a sedan.