Has anyone ever tried to fix the squeaks in a nail down installation by drilling holes in the subfloor from underneath in the squeaking area, and shooting some adhesive/etc in there, or just adding a few screws that go from the bottom of the subfloor into the wood flooring? It this possible?

Yes, thats been done before.

If the squeaks are out in the field you might also try driving wood shims between the subfloor and floor joists to tighten everything up.

There is another possible solution if the squeaks are near a wall. The bottom plates are sometimes not nailed down good and ride up and down on the subfloor. If they are near a wall remove the baseboard and nail at an angle through the bottom plate into the subfloor, there should be doubled up floor joists under the walls. Use long framing nails with a ring shank like 16d treated wood nails.

Good luck.