You could always fit a chuck lock of the type discussed in this thread when running in reverse:
Chuck Lock by Woofity
Similar chuck locks were supplied for their screw-nose lathes by respected companies such as Schaublin so they're not gimcrack devices. This assumes, of course, that your chuck backplate has a spigot 'neck'.
Incidentally, Woofity's version is a little more elaborate than commercial versions. The latter were commonly a simple split collar with a single screw spanning the gap (the strain relief hole opposite the split is still required, though).
Joe
Edited By joegib on 13/11/2012 06:55:12