Hey Lee A.
With regards to the JAA atpl groundschool, no problem doing it in an approved school in the states with the minority that offer it, infact 1 FTO offered it up until recently and now I believe another FTO will carry the flame SO just the 1 FTO. However I am pretty sure you will still have to go back to the U.K to sit the exams, the same applies if you do the groundschool with the South African FTO who offer it, you must do the exams back in U.K.
In relation for the FAA ATP, you cannot gain that until you have the 1500hrs already logged, unlike JAA where you can get the FROZEN atpl with just a ppl. You then unfreeze it by gaining the hours, just different ways of going about it but in essense you cannot be entiled to the priveledges of an atpl till your at the 1500hr mark. If you plan to get the green card and believe your in for a good shot, I'd milk it and go for the regionals over there and concentrate on your FAA ATP, If you want to keep your hand in the JAA-JAR (GET IT) do a distance learning course.