Will I be able to install a custom rom on my at&t GS4 with the MF3 update if i flash back to the old 4.2.2?

i understand i cant do it now because with the mf3 update the bootloader is locked but what if i install the original 4.2.2 firmware on it first?
thanks for the info. couldn't find anything. once i do root it, are there ROMs for it though?