Thanks so much, JDiver. I wasn't thinking clearly on the airside vs. landside issue. I would definitely prefer to avoid the craziness of LHR as well.
I guess I was just trying to find a way, if you had a 6 to 8 hour layover somewhere after flying all night, it would be nice to use a lounge somewhere if they had showers, even if it meant having to go through security again. The AC in FRA and LHR both have them, so I was trying to figure out if it's 'legal' to use the AC at the connecting airport if I fly AA transatlantic with the connection being on a oneworld carrier. (The examples given on the AC Admit Policy page seems to indicate that would be possible, but I'm not absolutely sure this would qualify.)
Thanks.