Hmm, I thought I had read at HoFo or one of the other forums that someone had it working, but it seems most folks haven't.
One guy said he was successful by combining an old LG adapter and a Moto adapter, but the YouTube video is gone, so not sure if it's real or not.http://forums.wireless.att.com/cng/board/message?board.id=LG&thread.id=33161&view=by_date_ascending&page=2
I think it uses some of the unused pins on the Micro USB connector (it has 5 conductors, USB uses only 4) or has some kind of short or circuitry to tell the phone that it's a headphone adapter.
The Xenon datasheet does
list a "3.5mm adaptor" so either the datasheet is wrong, or we don't know what the correct LG part number is and where to find it yet.