I am also looking for this solution. I don't think the other posters are understanding the need here. I want the user to have an option to save their file as read-only. I can add a check box for this in my main application, but it would be more elegant if I could just display a check box in the file save dialog.
My solution was just to convert my "Save..." button in my main program be a drop down menu button. If the user wants to save normally, then they click the button as normal. If they want to save as read-only, then they select the "Save As Read-only" option in the drop down. If that option is selected, then I make the file read-only in code, after it is saved.