I found what I needed with the following function, from http://bloggingabout.net/blogs/ramon/archive/2007/04/05/get-the-physical-path-of-a-path-that-uses-a-subst-drive.aspx
private static string GetRealPath(string path)
{
string realPath = path;
StringBuilder pathInformation = new StringBuilder(250);
string driveLetter = Path.GetPathRoot(realPath).Replace("\\", "");
QueryDosDevice(driveLetter, pathInformation, 250);
// If drive is substed, the result will be in the format of "\??\C:\RealPath\".
// Strip the \??\ prefix.
string realRoot = pathInformation.ToString().Remove(0, 4);
//Combine the paths.
realPath = Path.Combine(realRoot, realPath.Replace(Path.GetPathRoot(realPath), ""));
return realPath;
}
[DllImport("kernel32.dll")]
static extern uint QueryDosDevice(string lpDeviceName, StringBuilder lpTargetPath, int ucchMax);