@shady, sorry for the delay - had a UPS blow-up to deal with the last five days...
Anyway, I bought a 40m transmitter for each car, opened them up, replaced the batteries with name-brand ones, wrapped the battery pack in electric tape to prevent them from becoming dislodged, and the sealed the transmitters using liberal amounts of liquid electrical tape ($5 at the hardware store). I mounted the transmitters inside the front bumper (behind the plastic bumper, away from the metal frame). In both cars, the bottom of the bumper has cutouts in it, so I was able to ty-rap the transmitters to the bottom of the inside of the bumper. I stretched the antenna out (left-to-right across the bumper) and ty-rap or duct tape it down. I wouldn't place it inside the car, since the metal frame really interferes.
The plugin doesn't take long at all when it finds transmitters, so the only delay i see is the z-wave delay opening the door. I would stick with the 40m transmitters - you don't get nearly 40m with it through walls, and the RSSI receiver has shorter range if you go that route. I have a keychain version, but I haven't played around with it much.
As for serial setup, I think the final release of UI4 will automatically find the USB version of the receiver. I have not had a chance to update the wiki, but the latest version of the plugin automatically finds transmitters and creates devices for you, so we will soon be able to just plug-and-play.
I don't know what my plans are just yet. I placed a receiver in the bedroom, but what's funny is the keychain is found by the RSSI receiver in the family room while standing outside, but is not found by the standard receiver when standing more than 10 feet away. It also seems that the 40m is found by the stardard receiver before it is found by the RSSI receiver. So, I have some playing around to do. I don't know if the RSSI receiver will offer many benefits at this point, but I do intend to add "signal strength goes above/below" events... if I can.
One last note, I took @guessed advice, and bought a wiznet wiz112sr, and coincidentally enough, I can plug it right into the RFID board and fit it in the same enclosure - just a little bit of soldering involved. I just need to cut out for the network jack and power supply, but when I do, I will post instructions/pictures.