I have never seen such, and believe that such would be essentially useless.
An isolated ground receptacle is only potentially of use in an environment where there is current flow or electrical noise on the grounding system. For example, even in a _properly_ installed metal raceway system, you almost certainly will have loop paths described through the grounded metal. You will almost certainly have changing magnetic flux coupled through these loops, and thus current flow on the ground system.
I suppose you could also have electrostatic pickup by the ground conductor; a isolated ground conductor surrounded by a metal raceway would be shielded from this.
But in any of these metallic systems, the yoke of the isolated ground receptacle will be properly grounded through the metal receptacle box. No need for a second screw.
Now consider an isolated ground receptacle in a non-metallic box. Clearly the yoke is not properly grounded...but on the other side of the coin, a $0.39 Home Depot special in a non-metallic box essentially has an isolated ground. Use a non-metallic wiring method, and a single receptacle on the circuit, and if the run is back to the main panel, then you have something that is electrically an 'isolated ground' receptacle.
-Jon
