Any thing with an _ in the name should be considered a private variable and subject to change in future releases.
While many people have reached into some of these inner variables to make the widgets work for them, we in no means guarantee that future versions will work. Use at your own risk.
Our widgets can’t possibly handle all use conditions. We make them open source (http://github.com/coronalabs) so you can take the code and build it to fit your own needs. Also buttons are probably the easiest things to do with out widgets. Many people spend a lot of time trying to make widgets do special things when two images stacked on top of each other, with touch listeners that show/hide each other is probably less code than writing out a widget.newButton() specification.
Rob