Custom properties in props do not work

[gegenschall]

I'm trying to add custom properties to the props node, like so:

props {
    ItemDefinitionGroup.SomeVariable = SomeValue;
}

This results in the generated props file with the following content:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <ItemDefinitionGroup />
</Project>

Is this even a supported feature? If yes, how is it supposed to work or is this a bug?