Scratch previous post, I have a strong hunch I know what you need to do, from something loosely related that I just got working. In the super_affordances for object_terrain, try adding the interaction generic_ToiletStand (id is 151354 if that helps you locate/add it)
Edit: On giving it more thought, this may not be as much of a clincher as I thought. The reason it seems important is because I ran into a situation in experimenting with SitOnGround and attempting to allow ages other than toddler to do it, just to see if I could get past the contraint, for one thing. In the end, what clinched that was editing the age in both the SitOnGround interaction and the "generic" SitOnGround (there are a lot of "generic" named files like these, which are apparently important in interactions that involve special posture).
