Currently, arrows and bolts have both the Wood and the Steel types, allowing the special materials modifications that allow them to bypass damage reductions (i.e. adamantine, alchemical silver, and cold iron). But, darts, javelins and the spears do not, having just the Wood type. So, the Steel type could be added to darts and the spears.
If the concern, though, is not to have the weight reduction of mithral, perhaps the MITHRAL_ITEM equipmod could have a qualifier like "BONUS:EQM|WEIGHTDIV|2|TYPE=Enhancement.REPLACE|!PRETYPE=Wood" to exclude those items that are partly or mostly made of wood? [Edit: This part doesn't work, unfortunately, since adding the Mithral type replaces both the Wood and the Steel types. Hmmm.]
This is one I could do, if you approve it.
Completed: At revision: 20886
Added Steel type to javelins, spears (longspear, shortspear, and spear), and darts.