Bob, you'd have to make some type of brace to hold the antlers out of the water. I've done about 50 Roe deer and 10 Hirsch (Red Stag) and countless foxes and badgers when I was hunting in Germany for the past 5 years or so. I made a little vise on a flexible piece of metal pipe from a desk lamp to keep the horns out of the water. Of course a set of roe deer antlers fit in the palm of your hand and don't weigh a thing, so its a lot different, but the principle is the same.
There was a commercial powder available at the hunting sops in Germany used to do this....we cut out all the other steps there and go straight to the boiling with this stuff in there and it strips the flesh right off the skull. They also say use a little dish detergent while boiling cause it cuts the oils from the skin and meat.
Anytime we hit the antlers with the whitener by accident we would touch them up with a mix of brown and black shoe polish and you can get the shade to match just right.
Another trick for "bleaching" the skull when it has antlers or horns, whatever chemical you use is this.....
Set the skull in a baking tray with inch or two high sides. Wrap the skull inside and out with those cotton sheets (like cotton balls but rectangular) and attatch them tightly with rubber bands. Now soak the cotton in whatever you were going to use and it holds it the skull without getting it on the antlers.
Once you do this a few times, you'll find a good way to do it. I go staright to the boiling to loosen up (shrink) all the meat and flesh away from the skull and then use a screwdriver and a needle nose pliers and a good sharp knife to cut everything away from the skull and get it out of all the cavities.
Good Luck!