To me the UI seems to be the place to do it. Just need something that triggers the tip line to be active.
The whole idea is to make a bitcoin transaction more mainstream. Most restaurants in the U.S. don't bring the credit card machine to your table. Bringing a tablet over to the table tends to raise eyebrows unless the order was taken on a tablet.
Credit card usage took off because of restaurant use. Bitcoin needs that same acceptance in order to lock into the mainstream. That process needs to be as simple as possible.
Fixed that for you.
In most of Europe, the credit card machine is brought to your table.