Not that anyone is asking how Platina is as flexible as she is but I wanted to share images of one of my prototype skeletons anyway. :)
One of my recommended parts for use with this type of customized spine, whether you're buying one or making your own, is the DD2 waist (Normal | White).
(Because you would have to cut away at the outer skin to get flexibility, I don't recommend the DD3 or DDdy waist parts. Well, you don't have to cut at them but the whole point with this customized spine is giving your DD flexibility. DD3 and DDdy waist parts don't allow that flexibility without modification.)
The axle of the hip joint is shorter and slightly thicker than a Volks hip joint axle. The shorter aspect is irrelevant unless you're using DDdy parts. (DDdy has larger thighs compared to a regular DD. The thigh size gets in the way of movement so they tend to dislodge from the axle if you want your daughter to cross her legs.) The axle thickness was addressed using sandpaper so the DD thighs can fit perfectly.