It would work. Flashbios is 256k. You can flash it to each individual bank 5, 6, 7 and 8.
Take note though, when/if you flash another 1mb bios onto bank 5678 all 4 256k bios' will be gone. Just do like cat suggested and flash it once to bank 5678. The first 256k of your bank will be flashbios, the rest won't change. But if you boot from 5678 you will only ever get flashbios.