You last said "Good luck!" and I thought I was all set, but alas .. it
seemed to be loading ok, but then "Missing operating system" was all it
said, a horror story long preceding the infamous BSoD to old DOS jocks.
Hm. The image ought to have the boot blocks. Maybe a geometry
difference? If Win95/98 is easily available, it could do a 'sys' on
that USB drive (not MS-DOS, because it can't see USB). Or maybe
FreeDOS would work, I have not used it much.
For that matter, the FreeBSD MBR bootcode might be enough ('fdisk -B').
I have a 32M MS-DOS hard drive image on my PXE server just for this kind
of situation. Mount the image, copy the firmware update files to it,
umount and PXE boot the target system from it. A bit of a hassle, but
often less than the alternative. Most systems work with it. Although a
few have complained and refused to run the firmware update, no other
problems (that is, no bricks).