Unfortunately, Microsoft does not support performing an in-place upgrade of a Windows 7 Ultimate installation to a Windows 8 Enterprise edition; Windows 7 Ultimate can only be upgraded to Windows 8 Professional (since Windows 8 does not come with an Ultimate edition). True, there might be little added value for a home user in Windows 8 Enterprise, but since it was the only version I had ready on a bootable USB stick, I tried to fool the installer to continue anyway.
This was surprisingly easy. It suffices to modify the "EditionID" and "ProductName" registry keys in the following location:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version
from "Ultimate" and "Windows 7 Ultimate" to "Enterprise" and "Windows 7 Enterprise" respectively, to let the installation proceed.