That would require a full GTK installation in initramfs, which would be a bit heavy.

Ah, by GUI I assumed you meant something good indeed booted for hunting things like homes. For during the boot process for mounting an encrypted root, a TUI using something like dialog (https://invisible-island.net/dialog/dialog.html#screenshot and commonly available in Linux distributions standard repositories) would probably be more suited. IIRC it is what Debian use for their text installer. It should be pretty light weight, though it is still something you are scripting and adding to initramfs yourself so there may be more pre-made options that require less manual maintenance.

(hunting -> mounting - didn't notice until the edit option has expired)