I found the blocking issue. The golden image used for Brix was created with a mender-convert version that used the Grub 2.04 build and not the Grub 2.02 build that you kept referring to. After building my enhanced Grub 2.04 BOOTX64.EFI and copying that one to the Brix boot partition I now have a working Grub again and one with RegExp as extra command.
Now I need to figure out ho to use regexp for my bootdrive extraction bit!