How do I prevent IVsBuildPropertyStorage.SetItemAttribute from escaping special characters?

Question

I am working on a Visual Studio 2010 extension and I want to add an attribute to an MSBuild Item, as follows:

 <EmbeddedResource Include="SomeFile.xml">
      <FooAttribute>%(Filename)%(Extension)</FooAttribute>
 </EmbeddedResource>

So, far the only way I found is using the method IVsBuildPropertyStorage.SetItemAttribute . This works fine for simple strings, but when i try to use characters that are special to MSBuild, I get this result:

 <EmbeddedResource Include="SomeFile.xml">
      <FooAttribute>%29%25%28Filename%29%25%28Extension%29</FooAttribute>
 </EmbeddedResource>

This means that SetItemAttribute automatically escapes from MsBuild characters and I don't want that.