yes you can do a lot background UI interaction using the AX APIs. Displaying a second cursor is also simple, just a borderless, transparent window that moves around.
For the few things you cannot achieve with the Accessibility API's there are ways to post events directly to an app - even though CGEventPostToPid is mostly broken when used on its own. These require a combination of CGEventPostToPid and CGEventTapCreateForPid. (I have done a lot of this stuff in my BetterTouchTool app)
For the few things you cannot achieve with the Accessibility API's there are ways to post events directly to an app - even though CGEventPostToPid is mostly broken when used on its own. These require a combination of CGEventPostToPid and CGEventTapCreateForPid. (I have done a lot of this stuff in my BetterTouchTool app)