@Staehilomyces I don't know, mites have never been an issue for me. Perhaps when you are ready to sex him, you can scrape them off while he is unconcious. The paintbrush method might work while he is active, try to use one with stiff bristles. I have also seen one of Mastig's videos where he put a mite-covered R. longipes in pure sand, and when it burrowed, the mites were abraded off.
I'll probably opt for one of the first two. Now that it is docile, I probably won't even need to knock it out, at least for the mites. I'm certainly not doing the last option. No more centipedes in dry substrate. NEVER!!!