The ball on the bottom of the body idea is working out. Now when the polymer clay Wiggles bend it'll look like their shirts are tucked in. I can see using this on some other dolls too. One of the Doodles may have this tummy with the balls attached to her hips and the sockets in the thighs to keep the look of her costume.

This is leaning backwards, has slightly more range going forward because of the dip in the front of the hips. I need to refine the leg sockets before any more trimming on the top of the waist.
As for knees I'm going to try the caps on the thighs. If the sockets are right the weight of the doll standing upright can hold them somewhat in place.