What I do is drill two holes in a couple of slats of wood.  The wood
would be maybe 3/4-inch thick and an inch wide, and would be oak or
maple.  The holes are down towards one end of the boards and about
3 inches or so apart, far enough apart to admit the piano key between
the slats at that point.

The wood with the holes in it should be maybe a foot or so long.  I put
1/4-inch machine bolts through the holes with their nuts to make the
two boards a clamp.  I slide the warped part of the key (in your case,
the capstan end) between the bolts and I clamp the wood to the key by
tightening these two bolts.

Now you have the key clamped down towards one end of the boards.  You
have nine or ten inches of the boards that you clamped with free.  Clamp
those together with a small clamp.  Take the key and clamp arrangement
to the work bench and clamp the free end of the key to the bench or put
it in a vise, so that the clamped end of the board (the capstan end you
said was warped) hangs out over the edge about 6 inches.  Support the
key underneath somehow.  Take a weight like the small sledge for
driving tuning pins (that's what I use) and use it to weight the long
free end of the clamping boards so that it will tend to twist the key
in the direction you want.

Then I put a teapot on a hot plate (or other steam producing device)
underneath the key and watch the key for several minutes with the steam
playing around the warped part of the key.  Don't put the steam too
close to the key -- you don't want to soak the key with condensation.
You will see the weight slowing move down over several minutes as the
key twists in the other direction.

I always go just a little bit farther than what is called for because
I know when I take the steam away, the key will want to warp back to
where it was before.  But if you let it twist a little farther, when
it tries to warp back it will hopefully wind up being exactly where you
want it.  When you judge it's time to take the steam away, leave the
weight there as the key cools.

After about ten minutes or so of cooling, take the weight off, unclamp
your key, try it in the piano and see if the capstan has twisted to
line up more fully with the piano sticker.  The reason you support the
key from underneath is so the weight of the small sledge doesn't cause
the key to bend out of true .

I suppose instead of drilling those two boards you could just clamp
one 3/4" x 1" or so board to the key at the warped end.  The idea is
to weight the key and steam it so the warped part twists back to where
it's supposed to be, and support it underneath so it only twists and
doesn't "sag" out of true.  I've used this method before and it works.

Tony Marsico