Resolvemos o problema de maneira bastante hacky, adicionando o seguinte à seção de especialização do unattend.xml usado com a imagem do WDS:
<settings pass="specialize">
<component name="Microsoft-Windows-Shell-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<ComputerName>%MACHINENAME%</ComputerName>
<TimeZone>GMT Standard Time</TimeZone>
</component>
<component name="Microsoft-Windows-International-Core" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<InputLocale>en-GB</InputLocale>
<SystemLocale>en-GB</SystemLocale>
<UILanguage>en-GB</UILanguage>
<UserLocale>en-GB</UserLocale>
</component>
<component name="Microsoft-Windows-UnattendedJoin" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<Identification>
<Credentials>
<Domain>$domain</Domain>
<Password>$password</Password>
<Username>$username</Username>
</Credentials>
<UnsecureJoin>true</UnsecureJoin>
</Identification>
</component>
<component name="Microsoft-Windows-Deployment" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<RunSynchronous>
<RunSynchronousCommand wcm:action="add">
<Description>Force Time Resync</Description>
<Order>1</Order>
<Path>cmd /c w32tm /resync</Path>
</RunSynchronousCommand>
<RunSynchronousCommand wcm:action="add">
<Description>Force GPUpdate</Description>
<Order>2</Order>
<Path>cmd /c gpupdate /force /boot /sync</Path>
</RunSynchronousCommand>
<RunSynchronousCommand wcm:action="add">
<Description>Reboot</Description>
<Order>3</Order>
<Path>cmd /c reboot -r -t 1</Path>
</RunSynchronousCommand>
</RunSynchronous>
</component>
</settings>