Comment 6 for bug 1247933

grub-install --bootloader-id=customfoldername # <-- this is exactly what I wanted

it didn't work for me at first, but then I found this: https://askubuntu.com/questions/1129269/install-grub-using-custom-boot-loader-id-e-g-myubuntu - "... After running grub-install with --bootloader-id, run grub-install without any arguments. It will create an ubuntu entry. Delete it if you want to, but now your id is going to work "magically". Very annoying, seems to be an old bug. Hope I helps."