A.FLOOR,
With a 777, you cannot track a back course localizer using the LOC mode of the AFDS.
Seconded, our FCOM quite clearly states you fly a back course in LNAV, or TRK SEL or HDG SEL,
not LOC.
From our book: "Localiser Back Course. Do not use LOC roll mode.........If the Back Course approach is not included in the FMC database, insert the ILS frequency and it's front course QDM into the NAV RAD page; the localiser pointer will then display deviation in the correct sense."