I've never tried, but what about using a diamond-surfaced file? It supposedly works, although I'm not sure about the quality of the blade.
If it does work, and the pouch on the sheath is large enough, you could put in a micra or a p4, along with one of the skinnier diamond file sets from commandoproducts.com - I got one of thier screw driver sets from radiohack and I'm pretty impressed. At the very least, it could be tucked into a pouch with a full sized multitool.
If it doesn't fit as is, I seem to remember seeing someone offereing diamond-surfaced needle files online, I think maybe widget supply. Trim down the stem a little, maybe, and you are good to go.