Decided to see how bad the pass. side floor was, so I removed the carpet and sound deadening. Looks like water got under things at some point, and the primer put under the sound deadening panels is all but gone:
a bit of wire-wheeling later, this is the only spot that needs replacing (about the size of a postcard). The rest of the rust was just surface.
I was a bit surprised by that , as the subframe outrigger directly below the floor looks like swiss cheese:
I will eventually get around to removing the seats and all floor interior trim to check for additional rust (despite not seeing any from the bottom), as I came across this under the sound deadening (that's the floor/trans tunnel joint); just surface for now, but I want to eradicate anything else like that before it takes hold.