Bellerophon
If it is possible to slideslip in either direction in this condition, then it must be possible to find and maintain that position in which there is zero sideslip. Not me personally you understand, but perhaps a luckier pilot. It may be a difficult position to find, but that does not mean it does not exist!
Zero slip implies that the TAS vector is aligned with the longitudinal axis of the aircraft.
Hence, all aerodynamic forces from all non movable surfaces will cancel each other as far as their contribution to stop the rotation due to the engine failure is concerned.
The only thing remaining to stop the rotation (as far as I know Concorde), are the forces from the elevons, rudder and bank.
In order to fly without slip (and assuming the trust vector is parallel to the TAS vector) the aforementioned control surface forces will have to have to, when added together, create a couple (not a moment) equal to the moment created by the engines.
Assuming the horizontal component of the forces created by the elevons is small. The only option left over is to bank into the live engine and rudder.
The position that gives you a bank angle equal to the deflection of the slip ball should be zero slip straight flight on the condition tha your heading remains steady.