Its been discussed in here that as part of a RTW you can use the QF flight between JFK and LAX.

However - I'm being told I cannot use it as a last segment:

LAX --QF--> SYD --QF-> SIN --QF--> LHR --BA--> JFK --QF--> LAX

Phoned Qantas US ticketing, they say you can, then phoned Qantas Australia who say you can't - logic being than I didn't enter the US on QF.