I second the use of small screwdriver tips, which (if properly selected) can be very close to the desired mortise size, requiring only some additional shaping.

Another possibility is this "micro-knife" blade here, which I bought on a whim:

http://www.micromark.com/Micro-Blade-61-5

These are apparently used for optical surgery.  Although one would think the blade is therefore very thin and slim, it's actually surprisingly stout, and the edge is absolutely perfect in thickness and width without any modification.  I tried one on a scrap tongue I had lying around and it punched right through in one go with no trouble, making a lovely clean mortise.  I used it for over a hundred leather-to-delrin jack conversions on a first-gen Zuckermann straightside as well without issue.

The only issue is that it may be hard to chuck the thing into some kind of holder since the wider end is just a flat rectangle.  It's long enough to be held freehand (Jensen-style) if you want to use it that way.  The blade won't bend when tapped with a hammer.

BM

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