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 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.