I've also used oil (mineral oil, or even mineral oil-based hand lotion) on my DMT hones. In particular, it has made for better feedback on my Duo-Sharp bench hone. So long as the hone gets cleaned afterwards (dish soap & water), no harm in doing so. And the lighter the oil, the more easily it cleans up. Oil doesn't evaporate like dish soap & water does (in dry desert climate, where I am); that's the main reason I looked into using it, and found it to be useful.
I know DMT doesn't recommend it; but they also don't specify why. So long as the diamond hones are cleaned regularly, they're pretty much bulletproof as to what's used to lubricate them.
David