If not engineered in this fashion already, I would strongly recommend a direct GPS feed to your EGPWS.
I am fairly sure this is part of Egpws on the 744.
QF has modified all its 744 and 767 fleet to use the new "blue" honeywell EGPWS box and all its 744 (except 744ER) are all pre 2000 marks.
These aircraft have flown or do fly in US air space etc
So there must be a work around for your problem.