I ended up solving this using a custom inline task similar to the following:
<UsingTask
TaskName="GetMetadataTask"
TaskFactory="CodeTaskFactory"
AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll" >
<ParameterGroup>
<MyItemGroup ParameterType="Microsoft.Build.Framework.ITaskItem[]" Required="true" />
<MetadataString Output="true" />
</ParameterGroup>
<Task>
<Using Namespace="System"/>
<Code Type="Fragment" Language="cs">
<![CDATA[
StringBuilder command = new StringBuilder();
foreach (ITaskItem item in MyItemGroup )
{
command.AppendFormat("ItemName={0}\r\n", item);
foreach (string parameter in item.MetadataNames)
{
command.AppendFormat(" {0}={1}\r\n", parameter, item.GetMetadata(parameter));
}
command.AppendFormat("\r\n");
}
MetadataString = command.ToString();
]]>
</Code>
</Task>
</UsingTask>
Note that the above will also include all the default metadata that MSBuild automatically adds to every item in an itemgroup (like FullPath, RootDir, Filename, etc). In my implementation I added an extra check to ignore those metadata items that I didn't care about
Sample usage:
<GetMetadataTask MyItemGroup="@(YourItemGroup)">
<Output TaskParameter="MetadataString" PropertyName="YourMetadataString"/>
</GetMetadataTask>
<Message Text="$(YourMetadataString)" />
To see message output in Visual Studio's Output window, you may need to change your MSBuild output verbosity to at least Normal.