Alright, we are going to try the Flashrom method.
Please visit this article written by Fernando over at MDL:
https://www.win-raid.com/t455f16-Guide-H...-BIOS.html
Scroll down to the last section of the first post titled "Annex: How to dump the BIOS Region and to flash a modded AMI UEFI BIOS". Follow those instructions, but skip steps 8 and 9 (I have done those for you using UEFITool and a hex editor). You can also skip steps 6 and 7 if you wish (we already have a backup).
For the download, simply scroll down a little and find the attachment titled "Flashrom v1.2 Utilities.rar".
Step 5 allows you to boot into Legacy BIOS mode, which in turn allows you to boot into a DOS environment from a flashdrive. Let me know if you can't see a setting or option for a setting that mentions Legacy Boot/Legacy BIOS mode/CSM/anything similar. Whatever the setting is, if it is available, it is probably set to UEFI by default.
In step 10, replace "MOD.ROM" in the command with whatever you named the modified BIOS image.
As always, let me know how this goes and if you encounter any errors.
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Also, please feel free to PM me if I have not replied again about your BIOS mod request after 5 days.