However, Eris did manage to stun Quark with her chest fire for real.
I'd forgotten about that, you're right. (Although I think it was Sisko that got knocked on his ass, not Quark.)
Still, likely just rigged up with tech for that mission than any kind of "they just never use it" ability of the Vorta. The Founders don't seem like the type to just hand out extraneous gifts to their servants.