Other OSS projects use that model (I can think of Todo.txt Android app, for example).
But I'm not sure that it works exactly like "can you build it? It's free if you don't want to pay for it". OSS is about distribution. Anybody can build your app and distribute it legally (in this case Hacky is LGPL, it wouldn't be too difficult to comply with the license), even do it for a fee.
It's more paying for "the extras" straight to the developers. Call it support, iTunes, packaging, or just making things easier for people that don't want or don't have the time to get the hands dirty.