Gentle Giant - I am afraid you're out of luck. From the OneWorld rules posted earlier:
First and Business Class customers connecting on the same day of travel, or before 6am the following day, can access the lounge when travelling between an international long haul (a oneworld international long haul flight is defined as an international flight marketed and operated by any oneworld carrier with a scheduled flight time longer than 5 hours) and an international short haul or domestic flight (and vice-versa).
I think the problem with the return is that the international flight departs the day following your domestic flight departs (I am pretty sure they will look at the departure date of the international flight and domestic flights - rather than the arrival time).