Have you tried injecting more ink into one?
Howzbout cutting down a regular ballpoint refill, and sealing the cut end?
These pens have an integral; metal body which has to be the right size to
fit into the carraige mechansim of the plotter. 4 pens fit into a thing a
bit like the chambers of a revolver (as in the pistol) and the pen-down
mechanism presses on the back end of each pen. So the diameter and length
matter.
I haev pletny of dead ones here (I never throw anything like that
away...). Sometime I will dismantle a few (the first ones probably
destrctively until I can see what bits I need to save, etc) and see if
it's possble to refill them or use them to make holders for other
cut-donw pens.
-tony