We know the Founders impersonated key figures in their espionage efforts. We know they replaced several people from various governments.
With In Purgatory's Shadow
we learn who some of those people were. What I don't understand is why the Dominion kept them alive. The double was in place, they didn't believe in fairplay, and they really had no need to keep these Solids around anymore. Of course it's necessary for the writers to keep these guys alive, but in-universe, the Founders don't give a flying fish about them. Though I guess they might want to do some sort of mind probing experiments for intelligence as we'd seen before.
A Ferengi Changeling might have been interesting-- and nobody discovers him until "Rom" beats up Quark.
I liked the impersonated Commander Lovok of the T'alShiar. Too bad he blew his cover.