I seen a short viddy somewhere a few months back in which someone did the 'jiggle' with the girl wearing a t-shirt. Someone asked how he had done it and the dude wrote up a small tutorial. If I'm not mistaken, he did it the old-fashioned way... dials. Up-down-side-twist-etc. It may be more time-consuming, but it wouldn't hurt to try if you don't want to wait for a quick reply.

In fact, I've seen several Poser rendered clips where a girl 'bounces' or 'jiggles' and it looks very dynamic. It seems like they did something else besides using dials. But, it'll be a mystery until someone, out there, explains the secrets of the Jiggle.
:)