Not sure if your familiar with Vanilla. Another clean, very popular php forum software package. Here’s the link to the plug-in page that allows users to decide if they want the “Buy a Drink” (paypal donate) on each of there posts. Not being a programmer, I don’t know what it would take to port it to bbPress.
http://lussumo.com/addons/index.php?PostBackAction=AddOn&AddOnID=148
