Asking to tip after the user had played the game is a great idea, and I think it would generate a lot of tips for low priced or pay-what-you-want games. Maybe there could be a small inline "Tip the creator" button, under the thumbnail, beside the "Launch" button...? That would be quite discreet.
I don't know about limiting it to the app, though. Why not do the same thing on the "Purchases" page of the site.
Or... notifications. 24 hours after a download, players could get a notification prompting them to rate and/or tip. The notifications could be disabled in preferences.