Allowing users to have their own callback functions

It would be nice if we can have a way for users to attach their own on_complete and on_error function to the callback object.
At present defult on_complete and on_error are invoked.
Current callback create function prototype:
axis2_callback_t* axis2_callback_create(axis2_env_t **env);

what I would like to have is
axis2_callback_t* axis2_callback_create(axis2_env_t **env, void(*my_on_complete)(), void(*my_on_error)());