Since any horse can be crossed with any horse, yes, QH/appy is definitely a feasible combination. In fact, if you look around the forum, there's a couple of threads where our resident appy people get into discussion about the existence of "real" purebred appys anymore since they've been outcrossed to TBs and QHs so much. Many are of the opinion that the majority of today's appys are merely QHs with spots.