The default native way to make a custom (image only) tab bar (like the one on instagram) is to use UIEdgeInsets so we can reposition the image inside the Tab when it has no title text.
The best way for making this happen on Titanium is mapping a method and properties, allowing developers to set the Tab Item Insets for the Tab bar image (both for regular as for active images).
There is a property on UIBarItem (UIBarButton item inherits from this class) called imageInsets.
I suggest a Ti method on the Tab object named setImageInsets() in which developers can pass the absolute values for image insets and properly realign images inside the tabs. It would be ideal to have the imageInsets property as well so we can instance the tabs with the proper insets avoiding rendering flicks.
This is the best way to have a native tab bar without using the actual solutions of hiding the tab bar and creating a bunch of custom views on the bottom of the app or using unmaintained modules.