[claudius]

While certainly possible, I see two caveats here:

a) The ‘worthy project’ would have to license their product under GPL, too, which has the obvious effect that the whole thing is properly open-sourced again and everybody can slap ads on their stuff.

b) A license forbidding the modification to add advertisements is certainly not DFSG-free, and while this is likely of less concern in the Android ecosystem, I tend to measure the open-sourcness of a programme by its possible inclusion in Debian.