My thought when I read this post was: why not just use a mechanical pencil? They already exist, they're cheap, and if you don't retract the tip before putting them in your pocket the damage done to your pants is minimal. If you're loose enough in your definitions, a mechanical pencil is even doing what you ask for by only "feeding ink" (rubbing off graphite) when the tip is moving over paper.

The only disadvantages I see are that you occasionally need to advance the lead (not hard to remember, since it'll stop writing) and the marks aren't permanent (which can be advantage depending on the situation).