Hi Karl,
Someone here has put 8s in his doors, I believe they were RFs that were shallow enough, up front, so I figure you could do it in the rear doors too. I hear those Deflex Power pads work really well in conjunction with them.
I have a sport, and I was thinking the same thing; I wanted to up them as far front on that plastic wall as possible, but there's not too much space there. I think if you tried to get a box back there it wouldn't work (with the limited space), and if you tried just mounting a woofer it might work, but not sure. On the driver side, there is lots of space, but on the passenger side, there is some cluster of stuff behind the panel, so I figured there wasn't enough room. I just reached in the speaker hole and felt around though. Perhaps, since there is quite a bit of room, you could make some sort of spacer that will allow the sub an extra inch or so above the panel. I think you'd have to do some heavy fabricating, cause with 8s, the large plastic panels will rattle a lot.
Jon