I just picked up a new pen that has a tip that's 
(as far as I can tell) just as fine as 
photocopied lines.  It's a japanese brand I've 
never come across before, so its either a)rare or 
b)new c)not imported very often.  The brand 
is 'Itoya Finepoint System' and the pen says its 
permanent ink for writing & drawing on clear 
film, plastic, and glass.  It comes in different 
sizes and the .1mm pen leaves a mark about half 
as wide as lumocolor pens.  I've also tested it 
out, and it doesn't smudge/fade/stick etc.

I thought this is an especially good find for 
those of you who make correction layers for your 
faded cels since at lumograph super-fine tips are 
still wider than the original line you're 
replacing.