Hi Peter,
Narrow the problem down by booting any common Linux distribution (the install media is fine) and as root suspending with
echo mem > /sys/power/state
..and see if resume works there. If yes coreboot is not neccessarily off the hook, but xen/Qubes is at least involved.
Thanks for this suggestion. I have now tried it with a Fedora and a Knoppix live CD and neither of them can resume from suspend.
Kind regards,
Pete