You can just pass the custom command line as you run the kernel from UEFI shell prompt, e.g.

  fs0:linux.efi root=/dev/nvme0n1p1 initrd=ramfs.img loglevel=2

In my experience the nuisance part is creating and editing boot entries, especially if you try to set them up from the UEFI shell, so I tend to compile any initramfs and the default kernel command line into my kernel so I can drop it at /boot/efi/boot/bootx64.efi and minimise contact with the UEFI monstrosity.