I'd like to have the ability to have the rendering window's aspect ratio be locked to the aspect ratio of the Resolution set in the Capture tab. This could be a check box on Capture tab. This would help in framing out the view in the rendering window so that the captured "rendered view" would match the extents of what you see in the live rendering window.
Good news, not entirely what you want but close. The 2.5 will have the feature to show a "safe frame" before capturing. That will be the correct aspect ratio of your capture. This should allow you to perfectly frame your shot.