I don't know if it's possible using process start, but the following code opens the Windows explorer on the containing folder only if needed (if the folder is already open, or selected on another file, it's reused) and selects the desired file.

It's using p/invoke interop code on the SHOpenFolderAndSelectItems function:

public static void OpenFolderAndSelectFile(string filePath)
{
    if (filePath == null)
        throw new ArgumentNullException("filePath");

    IntPtr pidl = ILCreateFromPathW(filePath);
    SHOpenFolderAndSelectItems(pidl, 0, IntPtr.Zero, 0);
    ILFree(pidl);
}

[DllImport("shell32.dll", CharSet = CharSet.Unicode)]
private static extern IntPtr ILCreateFromPathW(string pszPath);

[DllImport("shell32.dll")]
private static extern int SHOpenFolderAndSelectItems(IntPtr pidlFolder, int cild, IntPtr apidl, int dwFlags);

[DllImport("shell32.dll")]
private static extern void ILFree(IntPtr pidl);