Seriously. Why should hundreds, thousands or several million (in near future :) users fool around with string options and a window only available if you activate "devel/debug" mode for such a simple task if it could be implemented to happen automatically with a few lines of code after pressing "Render only elements"?
And all that using a renderer trying to be as simple to use as possible? I´m also often in need of high res masks and noticed that DR(!) slaves seem to do the full job in that situation and was about to submit it as bug. Note it´s not about BB jobs where you have no GUI but about DR directly invoked from GUI wih the appropriate button labeled "Render only elements" :). Is there any (technical) reason for this behaviour?
Good Luck