Specify path to rule file using environment variable or macro?

Question

In some of my Visual C++ project files I would like to specify the path to a rule file using an environment variable or macro.

I have tried editing the ToolFile element with a text editor:

<ToolFile
        RelativePath="$(SolutionDir)\build\tao_idl.rules"
/>

But Visual Studio complains that it can't find C:\Path\to\ProjectFile\$(SolutionDir)\build\tao_idl.rules so it looks like it's not expanding the macro.

The same thing happens if I try an environment variable, using either the $(VAR) or %VAR% syntax.

NOTE: The project file is pulled into several other SVN repositories by using the svn:externals property, and not all the repositories that pull this project in use a standard directory structure.

Is this possible?