Chris;
Good question!
At one time, I was pretty sure that the ball-mount was just a big version of the well-machined thing seen on panzer hulls for the MG. From this, I would have said "yes, it would have been greased and it needed to be kept clean". I was, at one time, planning on finishing it accordingly on a Brummie build...
Now that I've become all growed-up and old and wiser (OK, I'm a dreamer and full of BS), I've become convinced that this was probably more like a cast-iron (cast steel) thing which was not polished nor even necessarily smooth - the pics seem to show it typically painted and matching pretty closely whatever color the vehicle was (hence my assertion about it being painted the base dunkelgelb). I no longer think that it would have been greased - something which would only be done if it were actually smooth and had a bearing surface...
But I would actually like to be wrong in this! I think it would look cool to have the ball be "naked metal" - mostly a steel color with some greasing on a build!
On the side - I think this same discussion would go around with that Sturmtiger... that, too, has a big ball-mount and maybe it, too, could have been a naked-metal w/ grease type of fitting.
In any case, this thing would indeed be a dust and muck trap! Which explains why many Brummie pics show a fabric cover in place. And a similar case pertains for the StuG... that mount was open to the weather, and often had a canvas cover behind the armor mantle.
Bob