Been there, done that
My advice would be to take your SA ATPL (or CPL) FIRST to the local FAA FSDO (Flight Standard District Office) in MIA. If you hold a current ATP, they will automatically issue you with an FAA Commericial (Restricted). No charge for this. However, because it is restricted, you cannot fly "for hire or reward" with this certificate. Then, take your FAA ATP written, combine the FAA ATP checkride as part of your 727 type rating and hey presto, you will come out of it with an US FAA ATP with a B727 Type Rating.
Good luck - it can be done provided you know the steps and it is certainly much cheaper than the UK CAA or now JAA/JAR.
Hope that helps.