all code is here :

https://github.com/BSzili/SDL/tree/mos-sdl2/src/video/amiga


When you call SDL_CretaCursor, all seem pass here :
https://github.com/BSzili/SDL/blob/mos-sdl2/src/video/amiga/SDL_amigamouse.c#L55

I try to look in SDK, we have an example in Intuition "argbpointer.c"

Thanks