Best Way To Load A Master Page From Outside The Current Application?

I'm prototyping an environment where multiple applications run on the same server, but they all share a master page. Is it possible to load a master page from another application? In one of my applications, I have this code:

protected override void OnPreInit(EventArgs e)
{
base.OnPreInit(e);
this.MasterPageFile = "~/../MasterPages/Root.master";
}

What this does is set the master page to a file sitting outside my current application. (Note the ".." after the tilde -- so I'm going to the root of my current app, then stepping up a directory to find a master page.)

ASP is displeased with this:

The virtual path '/MasterPages/Root.master' maps to another application, which is not allowed. I understand that it might be stressed out about the fact that there's no backing class for it, but what if I have nothing in the code-behind, and I choose not to inherit my master page from anything -- so the entire thing is contained in the ".master" file?Is there any way to do this?