Why not just try to get on a different PHX-LAX flight. Check the loads, see if you can somehow get on an earlier or later flight. Talk to the gate agent, etc... There used to be a bunch of flights a day, I'm sure there's still several. Or maybe try to go to SNA, BUR, LGB, ONT, etc if you're not locked into LAX.

Otherwise, if it has the same flight number, you'll be given miles MEX-LAX, and i don't think you'd be granted an exception.