It's a tier 1 customization to the AGKHelper.java file. Pretty straight forward. What I do is repurpose CreateAdvert() (initializes the Chartboost session), SetAdvertVisible(displays app wall or interstitial), and RequestAdvertRefresh() (caches interstitials although in my newest integration I just cache automatically when cb is initialized or an ad is dismissed).
It was a little tough to get going, but the folks at Chartboost were happy to help.
I have the java file for it on my site
naplandgames.com/agk but I haven't updated that for Chartboost 5.0.x which requires some minor changes.
I've got a few other customizations in there for analytics and install tracking. As well as proper method for minimizing the app on android (agk's end command is a fairly big no no as it terminates the app which should be left to the OS to do - Apple will even reject you for using something like that). The minimization method is also on my site. A very very simple modification.
Feel free to hit me up with questions via email if you need help with any of them.