This is a very innovative idea. Maybe smart tokens can be used for a tipping system? I'm not sure. Your token can be created on steem-engine.com. Then you could add funds to your coin to add value to it. People would buy it and sell it. Whenever you felt like a person deserves a tip, you could send it to them via transfer. Not by upvoting. That way you could give a tip no matter how old the post was. Why don't you try making your own smart token? You could implement your idea this way...