I also never clean my diamond stones and they work well. I would avoid a metal brush because the substrate that the diamonds are mounted in is a soft metal. A toothbrush should be perfectly safe.
The diamond particles are anchored to the steel platens with a nickel plating, which is pretty tough stuff. I don’t think a brass brush would be a problem. Certainly, a steel brush would probably be too harsh.
FWIW, I wash my diamond plates about twice per year using alcohol, just like I do with the diamond film, which I wash after two knives. For the first several years, I did nothing to clean my diamond plates and they did just fine.
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