You’re talking about trying that in /boot/boot/extlinux/extlinux.conf
I haven’t tried exactly that, but I’ve tried something using the uuid and it didn’t work.
There’s hope though, via uboot setenv settings, but I need a usb-uart dongle to try it carefully.
The nspawn is a very good alternative since you see the fedora 37 actually going through the init process. It’s better than chroot for sure.
The same approach could be used to run the debian sid from /mnt as well.