If all you want to do is remove the threads there's no need to rotate
the part. Clamp it in a vise and use a core drill of suitable size to
remove the threads.

Personally, I'd just wittle a dowel to match the bore of the threaded
end, grease it good, put on a pair of gloves and press the threads
against a 1" wide belt sander. The fitting will spin on the dowel
(and against your glove) whilst the abrasive belt removes the threads.

If you can determine the location of the axis of the threaded bore to
within a thirty-second of an inch or thereabouts you could also solder
a T-nut to the brass fitting, thread a rod into to the T-nut and chuck
the rod, allowing you to remove the threads with a file, sanding disk
or whatever. Finding the center isn't as difficult as it seems since
you can use the drill-press and a piece of dowel to spin the fitting
(by hand) whilst pressing it against the pointed end of a tack driven
through a board... even stiff cardboard would work. The trick here is
to first chuck a pointed scriber and use it to 'index' the point of
the tack (or whatever). Once the two were aligned simply clamp the
board to the table and go about setting up some method of chucking the
fitting. The tack will remain centered relative to the chuck.

Holding irregularly shaped work is a fairly common problem -- even for
people who have lathes :-) There is an almost endless variety of
solutions. The key point is that YOU CAN DO IT if you give it some
thought. (Humans are ingenious rascals. Sometimes I think we're
almost as smart as raccoons :-)

-R.S.Hoover