> Can you dual boot any other OS on ARM Macs yet?

Yes, https://asahilinux.org . I think this is coming at about the right stage in the game - the Asahi folks have figured out the baseline needs for booting another OS and have a basic bootstrap loader and hypervisor (m1n1) as mentioned in the M1 Windows writeup.

I think the OP writeup is fairly clear-eyed in terms of what needs doing - bring up the basic interrupt controller setup and IOMMU oddities needed to get UEFI and then the kernel to come up, and then start chipping away driver by driver until the system is usable.

This is basically what Asahi did and it's worked out well for them.