I experimented with an adjustable nut on my basses, but only to the extent of pushing a bit of galvalized wire under the strings next to the nut, so that it could be slid/angled as required.
Made a real difference to the intonation at the low end (bottom 3 or 4 frets), and after setting the intonation at the bridge has negligible affect elsewhere.
Didn't do anything permanent as I assumed that the required position would change with the strings fitted.
In princible a good idea.