The Xposed Framework (http://repo.xposed.info/) allows you to hook arbitrary Android functions. And APKs are pretty trivial to modify, though they can't be re-signed. Not sure if it's possible for Snapchat to prevent TOS violations, seeing as they need to put the pixels on your screen somehow.

I used Snapchat only briefly because it's horrible, but people enjoy continued success in saving dick pics etc. with the Keepchat Xposed module in current Snapchat versions.