NixOS on T400 with Hybrid Disk Layout via Disko

I am trying to install NixOS on an old ThinkPad T400 and have trouble getting it to boot. I use this disko config to partition the disk and this script for installation. The installation goes through just fine, but when I try booting it I just get a cursor blinking on the top left of a blank screen.

The T400 does not have a UEFI and therefore does not natively support GPT disks. As disko developers seem to discourage usage of legacy MBR tables in favour of hybrid configurations this is what I am trying to use. I am not sure, whether the issue actually is with the disk layout, but it is my strong suspicion, since not even a bootloader is shown.

Any help or hints are much appreciated!