I've explored this a fair bit, and this is much the same situation as F on the 762 routes. You'd need to be in paid business to be upgraded to F via SWUs, or miles + copay. I'd recommend seeing whether the business class ticket is within your price range before beginning.

Anecdotally, I was booked into first with miles +copay on a two class between LAX-MIA and they changed the flight to the three class 777. The system automatically rebooked me in three class F. This was quickly corrected (within 24 hrs), and I'm back in J, so they seem to protect the F cabin on these pretty heavily.

I agree with FWAAA, its one of the best uses of miles domestically.