"Preserve Transparency" might be one of the most important features of Painter. At least, I use it constantly, but surprisingly there's no menu item for it (that I can find) and no therefore no possibility for a keyboard shortcut.
Adding this to the Layer menu seems simple and (for me) very useful.
EDIT: The script is a clever solution to creating another button, but it's still not something you can bind to a hotkey. This should definitely be added to the palette menu section of the shortcut manager.