How about some kind of platform that you can retrofit to sitting toilets? That way if you wanted to squat you could pull out the platform and simply place your feet either side of the bowl.

I think the opposite approach would be much easier and simpler; install a squatting toilet, and then make some arrangement with a sitting ring that can be folded down when needed. In my head it's such a simple easy concept that I almost find it weird if it doesn't exist any toilets like that.