Sorry about the delay, I'm having internet problems.

Attached above is the text of a cr2 to demonstrate a variable joint center "CenterTest.cr2". This works in any version of poser from at least P4 up. It does not rely on extra body parts. The figure has 5 actors including BODY. You can ignore the hidden actors CenterIconA and CenterIconB, as they are only there as a visual representation of where the joint center is, they play no active part in what is happening, and could have been left out. PartA and PartB each have a dial "CenterY" that you can use to adjust the center of rotation for the respective parts. To keep things simple, the center is only adjustable on the y axis, though it could be done for all axes if desired. By default the joint centers of each part are in the center of the respective part, and the figure uses Wireframe Display Style.

PartA and PartB each have two extra translateY channels named yTranA and yTranB. ERC is used to link the extra translate channels to the "CenterY" channel-dial in such a way that when one translate channel goes positive the other goes negative. The main trick to the whole system relies on the fact that one translate channel comes before the rotate channels, and the other translate channel comes after the rotate channels. In essence, the actor is translated, then rotated round its new position, then translated back to its original position. This system, or something very like it was first brought to light by VK.