Michael is no stranger to the same problem. It's actually a Poser problem, not a specific figure problem. At first glance, it might seem a figure isn't affected... Make an adjustment to an ERC channel on the first figure that uses that naming convention, then select a group to alter manually or apply a pose from your library on subsequent like figures in the scene, and viola... cross-talk rears it's ugly head.

My hopes are that Poser5 makes the use of a null to load the figure through... obsolete.

With MBM, it's no different than any other figure... again, it's a poser problem. When poser creates the pz3, it writes figure names as they were in the scene. The ERC channels however, all point to Figure/instance 1. So, your faced with the same problem in the scene had you not used the null to originally load the figure.
Can it be fixed? Yes, albeit it must be done manually. What needs to be done is the ERC channels on subsequent figures need to have where they're pointed, fixed. That is, on "Figure #" (with instances of the same ":#") the ERC channels need to be redirected to its respective host.
Until a better solution is available, you could either use the one above or save individual poses from the scene to your library and, using the null, rebuild the scene... I know, not the desired route, but sometimes you're limited by the tools.

-Rob
rbtwhiz.com