Booting in this way has a number of substantial advantages over the way
the Raspberry Pi boot process currently works:
* The kernel command line can be modified by /etc/default/grub
I'm a (huge) fan of ZFS and run my X86_64 Debian hosts with riot on ZFS. I've taken a stab at implementing this with Debian. It's been done with Ubuntu but I was unable to determine what I needed to do to accomplish this with Debian. I wonder if your approach would make that possible.
* Multiboot installations on the Pi become possible.
I'm also interested in this. I've started work on this at
https://github.com/HankB/pmb which is a brute force approach, swapping the contents of the /firmware/boot partition between installations that reside in other partitions on the media. Other stuff has bubbled to the top of my priority list and I have not worked on this in a while but perhaps it might be of use to you or give you some ideas. I think that it's a worse kludge than your suggestions and should be nowhere near tha Debian repos, let alone upstream.
best,