Windows Server 2008 install: wpeinit -unattend:unattend.xml does nothing
Thanks for the suggestions, redblacktree, but the XML file was in fact well formed, and it was working fine off a USB key.
I found out I could use /unattend with setup.exe, so I copied the entire Windows install DVD to a file server, booted the same WinPE image via PXE, then ran:
net use w: \\server\share
w:
cd x64
setup.exe /unattend:w:\unattend.xml
And it worked fine!
The other gotcha is that I had to run
wpeutil InitializeNetwork
before I could see the network.