I just got home from working at my shop (it’s about 12:30 AM) so
this may be a crazy suggestion to someone whose brain is
functioning, but how about drawing the design with a graver,
drilling a hole every so often, and then using a slim reamer and
maybe some ball burs to cut away the rest. You didn’t mention
how thick the metal was so I don’t know if the ball burs would
last very long.
If the pattern is open enough, and the material is not too
thick, you may also be able to take a broken saw blade (4 or
larger probably) and secure it into an exacto knife handle so it
only protrudes maybe an inch, max (never tried it, just thought
of it) and use it like a keyhole saw. A short length like that
might be stiff enough to work with.
I think any of these methods will be somewhat tedious, but who
knows, they may work. LOL!