You need a left handed milling cutter. I didn't know they existed, but I found this fairly quickly:
I don't know how easy it would be for you to get them, or how suitable they might be. They all look a bit long and skinny to me. And would you then have the problem of the cutter/holder unscrewing?
If you are cutting brass you could make your own cutter from silver steel (drill rod), but that depends on the sort of cutter you need.
An alternative would be to add an extra locking device to the chuck, perhaps locating in one of the tommy bar holes - if there is some where convenient to anchor to on the rotary table.
Without knowing just what you are trying to do it's difficult to know what to suggest. Can you post pictures or drawings?
