What about drilling out the innards of the pen, then installing the cut-down
guts of a fibre-tip ("fineliner") pen in its place?
You'd probably need a drill press and some form of mounting jig to get the
The easy way to do that is to clamp the pen body in a lathe chuck and put
a twist drill in the tailstock chuck. I would think most self-centring
lathe chucks would be accurate enough for this -- easily.
The only problem is that most, if not all, lathe owners are not going to
want ink all over their machine tools...
-tony