Hi @KaosEngineer,
Sorry for the ~1 year lag... I left off this in frustration then, but want to come back to thank you. Your last post mentioned about the need for the SD AV cable and that actually did the trick - the "blank screen" attempting to boot the backup/recovery FlashBIOS 3.0.1 was due to my use of the HD component cables (duh).
Once that was resolved, everything else was "as expected" - I managed to "erase"/overwrite the 2x 1MB 3924 with 4x 256KB ROMs, then re-flashed 1MB bank #1 again with 3924 and my Xbox was working again, replete with all my saved games, profiles, et. al. still on my HDD...
Thank you so much, @KaosEngineer!