arm64 metal instances not booting?

[Jeremy Drake <jeremydrake+debcloud@eacceleration.com>]

Hello,
I am attempting to launch a c6g.metal instance in us-west-2, but it hangs 
up and never seems to boot, with:
 ami-0a773686fc288ee18
 ami-054ec2aaf512d49c5
 ami-0bb3ac1447646faf5

I also tried with m6g.metal, same result. I tried launching a more 
'normal' arm64 instance and that was fine.

The wiki says that arm64 metal support was added in with the 10.7 update 
(as of December), so it seems like it should work.

I am at a bit of a loss on how to diagnose this, none of the 'monitor and 
troubleshoot' options seem to work - get system log is blank, and serial 
console and get screenshot give errors.