For the record: update.
Windows Server is taking the "laptop" status from the ACPI. By instaling Windows "manually" (F5 during installation - when the system prompts you to hit F6 if you want to install extra scsi drivers), you'll be presented with a menu that offers you a number of options for manually setting up single/double processor, ACPI, and more. The OS installs itself in the "mode" you select. By disabling ACPI it will never know if it is on a laptop or on any other machine, and Dynamic Volumes are back.

Now I have to find a way to disable ACPI without reinstalling the OS. If there is one...