Quote - And there seems to be absolutely no way to create a chain of body handles which all deform the same actor.

Not sure what you mean by "a chain of body handles". It is certainly possible to have a number of body handles all affecting the same actor, though there may be asymmetries where the influence of two body handles overlap. But what do you mean by "chain" in this context?

I may be wrong, but in terms of emulating muscles, I doubt that body handles, or ghost actors, would be a very practical proposition in Poser. Magnets sound more promising, and there is no reason I can think of why they can't be driven by ERC, in much the same way as JCM works. In many respects magnets would seem to offer a lot more flexibility than JCM, as the Mags, Bases, and Zones, could be moved in many ways by inputs from joint rotations. Also it is easy to extend the influence of magnets on a figure to its clothing, by the application of a pose file. The magnets could be hidden from the interface, as in V4. On the down side, only fairly simple deformations are practibale with magnets, due to the limitations on Mag Zone shape. For detail in muscle sculpting, morphs are still the only practicable method.