Perhaps a clamp-on style quill stop would mesh threads enough to stay where you put it. Since it clamps on, the threads wouldn't have to match perfectly.

We actually just got a Delta 20-950 and were not particularly impressed with the fit of the stop nut: it seems a bit sloppy and lacks a real POSITIVE stopping feel.

http://www1.mscdirect.com/CGI/NNSRIT...MT4NO=58876283