Good call about the extra tips. They should work no problem in a lead holder, I would think. The lead holders I've seen have metal claws that stick out through the tip to grab whatever lead you're inserting, so perhaps they're more forgiving about size than a mechanical pencil that requires a particular size of lead.

When I get one for version 3 of the iPad stylus I want to make, I'll make sure to grab an extra one and use one of my extra stylus tips from this pen here.