GRUB Standalone with embedded config file and different keymap

It is possible to create a single GRUB EFI binary without any additional config files, modules, drivers, themes etc. on the boot partition. All those files will be put in a tar memdisk and then included into the binary itself, creating a "standalone" .efi file.
While that isn't really necessary for most users, it can be beneficial for full disk encryption scenarios.

GRUB is the only boot loader that can decrypt a LUKS blockdevice as of today.
However, in the standard configuration GRUB places its config files and drivers onto the boot partition. But on a fully encrypted disk, those files can't be read at boot.
Consequently no customization (e.g. changed colors, background, screen resolution) is available and one is greeted by an ugly password prompt before one can even chose what kernel or OS to boot.
Also, the decryption takes ages, even on a modern CPU. If you just want to boot a different OS on a dual boot machine this can be very tedious.