OK, did a bit more digging and came up with this:
<ItemGroup> <Namespaces Include="myns"> <Prefix>myns</Prefix> <Uri>http://schemas.microsoft.com/developer/msbuild/2003</Uri> </Namespaces> </ItemGroup> <MSBuild.ExtensionPack.Xml.XmlFile TaskAction="UpdateElement" File="%(SSDTPublishFiles.Identity)" Namespaces="@(Namespaces)" XPath="//myns:SqlCmdVariable[@Include='DeployTag']/myns:Value" InnerText="$(CCNetLabel)"/>
which worked :)