There has actually been some interesting change of language on the oneworld web site. At the moment, it does appear that the guest of an elite member no longer needs to be departing on an oneworld flight.

Here it the discussion thread that tracks the language changes over in the oneworld forum:

http://www.flyertalk.com/forum/onewo...ge-access.html

Personally, I have been guested into the AA LAX First class lounge on arrival (the agent did not check that I had no departing flight).