You might regret using a "soft dremel bit"
I use abrasive rubber bits to remove material from feed ramps. They can be quite agressive. Try the steel wool and polish idea first. They remove very little material and all you want to do is remove what is quite likely a small rust bloom
x2. Those Cratex-type abrasive wheels WILL remove metal. If it's just rust, 4x steel wool and a bit of oil shouldn't do any harm. Actually, you could do this with your hand drill with the wool wrapped around an old bore brush or on the end of a bit of dowel. If you really need to polish the metal, despite Chinbulco's caution, I still think a felt bob and some very fine abrasive (1000x or more) might be the way to go and won't remove enough metal to affect the dimensions of the die. A couple of microns difference won't throw your reloading off.