Are there full scale helicopters capable of sustaining inverted flight?
There is no real helicopter capable of sustained inverted flight and certainly not of hovering inverted.  

The simple reason is that there is no operational need to do so.

Is it theoretically possible?  Yes, but you have a lot of engineering problems to overcome.  In addition to inverted fuel and oil systems, the rotor head will be much more complex than it needs to be.

When flying normally, the fuselage effectively hangs like a pendulum underneath the rotor with a large nut (OK, it's more complex than that) holding the rotor onto the rotor shaft hub.  The rotor pulls the fuselage up.

To fly inverted, the rotor must be able to push the fuselage up so the hub would have to be engineered to apply and withstand the appropriate forces in both directions.

Then, the rotating blade cuffs, and associated hinges, must be designed so that the blades can have negative pitch (as seen from the normal attitude) across the full range.  In turn, this means that the collective mechanism must be designed so that it is possible to set positive and negative pitch as required.  

The blades would also need to be more rigid, so that they do not cone excessively and contact the fuselage.  This would add more forces to the head joints which would need to be strengthened.

Finally, you must "reverse" the cyclic controls so that the expected collective pressure, cyclic back; nose up; reduce speed; works in the opposite sense.

I've probably not considered other factors.

All of this would add significant complexity and weight which, given that there is no requirement for inverted flight, will not happen.

By the way, the power required to fly or hover inverted would be the same as for normal flight.  There is no intrinsic reason why inverted flight would need more power - blades rotating at a certain speed, with a particular angle of attack will generate the same amount of lift no matter which way they are pointing.

The reason model helicopters can do this is that the ratio of the weight of the rotor to the weight of the fuselage is much lower than for a real helicopter, and therefore requires a lot less power to produce the required lift, and models have a much higher power to weight ratio.

The reason real helicopters can go inverted for a short time in aerobatic manoeuvres is because the momentum on the fuselage is pulling away from the rotor so that the lift vector pointing up, toward the fuselage, is countered. Consider an outside loop, which would be impossible, since the momentum of the fuselage and the lift vector would combine.

You should be able to demonstrate this in a model with a normally designed rigid head.  Add some (secure) ballast in the fuselage until you must accelerate in ground effect to gain translational lift in order to be able to climb out.  Now try to fly inverted!