Would this be acceptable? (Kills the "#if !PSM")

    // We do not generally accept absolute paths here.
    // But ... PSM SDK requires absolute paths.
    // ensure any absolute paths start with Local
    bool invalidPath = 
        Path.IsPathRooted(safeName) &&
        (string.IsNullOrEmpty(Location) || !safeName.StartsWith(Location));
            
    if (invalidPath)
        throw new ArgumentException("Invalid filename. TitleContainer.OpenStream requires a relative path.");

I'm not 100% sure how well this would work across all platforms, but I think it keeps with the spirit of paths being relative to the content.