The flag is mostly analyzed with FEBuilderGBA.
In FE8, 0x128-0x12C is free, so I think that you can use here.
Global flags are 0x65 to 0xA4 for FE7 and FE6.
Global flags are 0x65 to 0x12C for FE8.
However 0x65 is reserved for game over, so it will effectively become 0x66.