Claro, faça isso no passe "specialize":
<?xml version="1.0" encoding="utf-8"?>
<unattend xmlns="urn:schemas-microsoft-com:unattend">
<!-- snip -->
<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">
<TimeZone>Eastern Standard Time</TimeZone>
</component>
<!-- snip -->
</settings>
<!-- snip -->
</unattend>
O TechNet tem detalhes adicionais para o componente Microsoft-Windows-Shell-Setup e Fuso horário .