I'm also sure you can do it with C4.  I seem to remember doing it in RayDream 5.5, actually.

My work approach is to build the metaball.  Go back to the assembly room.  Move to a later point in the timeline.  Double click the metaball to open the modeler again, and move the elements in the modeler.  Go back to the assembly room and you'll see a new keyframe in the Metaball. 

It works quite well, actually, for simulating liquid-ish shapes like the pseudopod in the Abyss.