The last days I tried to find the reason why I wasn't working and I think I've got a clue. I wondered why libGL.so.1 was needed after compiling Kivy against GLESv2. Well, it wasn't Kivy which wanted to load libGL.so.1 (even the -dev package was not installed on scratchbox), it is one of it's dependencies (I think SDL or even pygame) which needed libGL.so.1 and not libGLESv2. Thus, when importing Kivy to my code, OpenGL and OpenGL ES were loaded at the same time.
I decided now to rebuild everything (python, SDL, pygame and kivy) in a prefix. Got almost everything together, just smpeg is doing trouble, while ./configure'ing. It will be the last dependency for pygame which is missing in my prefix at the moment