I'd be willing to pay to have this developed also but I don't think it's possible since outside devs don't have access to an API etc.
What I'm looking for is this. A pop up window that is always on the Ipad screen. It would be customizable. So I could have it say "Ask the customer if they've tried our new candy". Or "Remember to tell them about our newsletter etc." In essence the employee couldn't ring up the sale until they hit the Ok Button on the screen. This would help myself and my employees add on sales.
Fellow retail businesses please upvote this to show shopify how much you'd like this feature. It would be pure money for retailers.