It seems it wants something like /iommu@0,10000000/sbus@0,10001000/espdma@4,8400000/esp@4,8800000/sd@2,0:b as the device
Weird:
device[sd(0,2,0):b] ("halt" to halt): cdrom boot (press RETURN to try default list): netbsd Booting netbsd
device[cdrom] ("halt" to halt):
It prints no error, but does not do anything either.
It tried to boot from the cdrom but didn't find anything it liked there. Or it failed to expand the alias. I seem to recall that Solaris etc use the comma-separated args in the boot command rather than space-separated args. Perhaps NetBSD is looking for something similar.
I think you're going to have to go grub around in the NetBSD code. :)