Surprised nobody's mentioning shellac in this thread.
4-6 coats give more of a natural wood with matt-ish appearance, but nobody's stopping you from french polishing it to rich deep gloss.
By far the best looking & feeling finish one can even put on wood.
It will wear. Some can call it patina and be happy about it, others can send the pen back to you (or any other woodworker who can do French polish) for a quick re-polish.
If it's dense hardwoods, there won't be nearly as much wear as on softer stuff like mahogany and walnut. Porefilling can be a pain, but we're only talking about a tiny pen, and pores are natural anyway (if they don't want pores, why not get acryllic/corian?).
