Here’s how you edit the (second stage) grub menu:
Just add your parameters after $cmdline_args and press Ctrlx or F10. This should have the side effect of the TPM measurements of the kernel command line being wrong and you will need to enter the recovery key.
BTW, that still doesn’t work.
$ cat /proc/cmdline
snapd_recovery_mode=run console=ttyS0,115200n8 console=tty1 panic=-1 quiet splash snapd.xkb="us,pc105,altgr-intl,"
$ snap get system system.kernel.dangerous-cmdline-append
intel_iommu=off
That is just after I’ve rebooted. I’ve also tried the system.kernel.cmdline-append but that fails, probably because it’s not on the allow list, or because of the pc-gadget snap; I think it’s the latter.