you could mod (insert as is on mmtool v5) the rebar dxe (both rebar dxe and nvstraps rebar dxe if you want), and try flash it using flashprog, as long as your bios is below agesa 1.2.x.x, if downgrade is possible, then try to downgrade to the minimum bios for your cpu, mod the latest one, then use flashprog to flash it, also remember to backup your current bios using flashprog before flashing the bios.
You could read about flashprog here.