A careless houseguest left a metal-bottomed can on the back of the
toilet--the very new, very expensive toilet. As might be expected, the
combination of moisture in the bathroom and the metal can left rust marks
on the toilet lid (not seat). I've done all I can with gentle methods (I
used dish soap and a soft sponge) and got rid of most of it. However,
there are still a few small, but noticeable, rust stains. I'm hoping for
advice on how to remove the rust without harming the ceramic. Thanks in
advance for any advice and/or suggestions!

Naval Jelly or some other oxalic acid based product. Takes very little.