I thought that might be a problem, since this BIOS is non-standard! The -savemac is what causes this, please be sure you get a working verified backup before doing anything else, that way you don’t end up loosing the MAC address and other PDR info I you flash.
It can be put back later with either BIOS edit or other tools. Try the BIOS region only, not full BIOS, minus the -savemac switch
You could also try making a copy of the BIOS file you want to flash, and then what happens if you change the file extension to .hdr and flash like you normally would with a Dell BIOS.
Do you even have a normal “Dell Update” type BIOS utility for this board, or only what’s originally contained in the exe BIOS file itself?