Here is one solution, it may not be the only or best solution but it is quick and it works.

Select the body part.
Open the joint editor.
Select the center from the top drop down list.
Make a note of the orientation angles.

With the body part still selected go to the Poser menu Object > Create Magnet
Select the new magnet base and rotate to match the body part orientation.
Select the magnet zone and scale it up really big. It is possible to edit the fall off zone to achieve a true constant but scaling is almost as good and quicker.

Now translating the magnet will translate the body part along its axis.

You can tidy it up by adding ERC dials in the forearm to move the magnet.

All of the above could be scripted.

Plan B would be to "do the maths" and add ERC dials and controls accordingly.