Here's a thought involving picks. Maybe, you could add a reaction to the Magma Generator (since it's stolen from dwarves) that requires, say, a steel bar (since kobolds can't make them on their own) and a pick (which kobolds can't use) to create a handpick, which kobolds could use to cut through stone. It seems like it would be sufficiently late game to not crash the balance, since it has pretty strict material and theft requirements...
Or alternately, you could add handpicks to the list of moodable items (which would make obtaining one entirely luck based) or the list of stuff stolen from dwarves, but at very low success rate.
Just a c-c-c-c-crazy idea.