Hi Ben,
Best way to do this that I can see would be to use, not
mpcorb.dat, but skymap.dat. This file is on the same MPC
ftp sites (listed at
http://www.projectpluto.com/mpcorb.dat ),
but contains only numbered asteroids. There are now about
50 000 of these. Matches up nicely with the constraint for
which you're looking. Download it and rename it to mpcorb.dat,
and you ought to be in business.
(And I probably should extend the ability to "filter" asteroids,
described at
http://www.projectpluto.com/update7.htm#asteroid_filtering
to include the ability to filter on number as well as on
orbital elements. That is, one could say
ASTFILTER=n < 50000
to get the situation Ben wants.)
-- Bill