I don't actually think you need msbuildextensionpack. You can do it just by using msbuild.
Example:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
<Target Name="UpdateGuid">
<PropertyGroup>
<_xmlFile>guid.properties</_xmlFile>
<_newGuid>$([System.Guid]::NewGuid())</_newGuid>
</PropertyGroup>
<XmlPoke Namespaces="<Namespace Prefix='msb' Uri='http://schemas.microsoft.com/developer/msbuild/2003'/>"
XmlInputPath="$(_xmlFile)"
Query="/msb:Project/msb:PropertyGroup/msb:GUIDProperty"
Value="$(_newGuid)" />
</Target>
</Project>