One of the big frustrations developers are quick to point out about the Android platform is that its app store uses the obscure Google ( NSDQ: GOOG ) Checkout system for payments. Recognizing this hurdle, mobile payments company Zong is announcing today a new way for developers to integrate carrier billing into their applications. While the offering will be enticing to developers, the one big question is whether Google will allow it. There’s no explicit wording in Google’s user agreement that prohibits it, and since Zong is all about enabling in-app purchases, rather than selling the apps themselves, it may have found a loophole. But it’s even unclear to Zong whether it will be allowed: “I think someone has to push the limits here to see what happens,” said Zong’s CEO David Marcus in an interview with mocoNews.