I see that domestic USA flights won't get lounge access. However, what if it's a domestic leg on an international ticket? For example at LAX heading to JFK from MEL.

Its not just food and showers, its internet access (which is woeful at LAX).  We were let in at around 10pm, so one strategy might be to wait until the 9:45 departures leave the lounge.  It would be a better client relations exercise to take volunteers for the $50 rather than turn away ...