Booting Windows VHD with custom boot flags

[DavidSpivey]

I have a driver I need on my Windows to Go on VHD instance that is not signed. In order for me to use it, I need to enable testsigning, which is an option in the BCD. ventoy_vhdboot.img inlcudes a BCD file that I assume I need to modify in order to add the testsigning option. However, when I use bcdedit to change the BCD, it tells me it can't find the file (This is from an administrator prompt). It can read the BCD but not edit it.

Code:

C:\WINDOWS\system32>bcdedit /store G:\ventoy\BCD

Windows Boot Manager
--------------------
identifier              {bootmgr}
device                  unknown
description             Windows Boot Manager
locale                  en-us
inherit                 {globalsettings}
default                 {default}
resumeobject            {5f7bf00c-9ab2-11eb-9a06-001a7dda7113}
displayorder            {default}
toolsdisplayorder       {memdiag}
timeout                 30

Windows Boot Loader
-------------------
identifier              {default}
device                  locate=\Windows\system32\winload.exe
path                    \Windows\system32\winload.exe
description             Boot from VHD
locale                  zh-CN
inherit                 {bootloadersettings}
allowedinmemorysettings 0x15000075
osdevice                locate=\Windows\system32\winload.exe
systemroot              \Windows
resumeobject            {5f7bf00c-9ab2-11eb-9a06-001a7dda7113}
nx                      OptIn
custom:26000027         Yes

C:\WINDOWS\system32>bcdedit /store G:\ventoy\BCD /set TESTSIGNING ON
An error occurred while attempting to reference the specified entry.
The system cannot find the file specified.

I managed to use Visual BCD Editor to modify the BCD to add AllowPrereleaseSignatures to the BCD "Boot from VHD" option. However, I now cannot insert the BCD in the ventoy_vhdboot.img file.

I have tried 7-zip and WinImage to add the BCD back to the image, but the errors say either the operation is not supported or the file is full. What program do I need to be using to modify the img?