Just buy plain brass, unplated beads. They still sell them. If they haven't tarnished enough by the time you wish to use them, you can choose from a number of methods for advance aging brass.

Re: plated beads...
On a large item, it wouldn't be too hard to dull the finish (or remove it entirely) with an abrasive polishing agent. Not practical with small beads. Instead, I'd say you're on the right path with a chemical treatment. My first thought is an etching solution. A cheaper alternative (free if ya got it!) would be oven cleaner. I'd put a few beads in a sealable can, hit it with oven cleaner, close it up and put it out in the sun for a while.