Maximum,
Sorry, I still can't see it. I'm not trying to be argumentative, honest! I'm a PPL with no multi-engine experience at all, so I'm just intrigued...
I can see why the aircraft would still be slipping slightly - if it weren't, there would be an unbalanced sideways force from the rudder. However, that doesn't seem strictly relevant - as far as the balance ball's concerned, the aircraft could be going backwards, but if (a) the wings are *exactly* level and (b) there are no other forces / accelerations involved (ie straight and level flight), then the balance ball *must* be centred, surely?
I would have thought that if the aircraft is slipping, even slightly, then the wings must be off level, abeit equally slightly - that's the definition of a slip. In the situation you initially described ("the ball will be slightly out towards the live engine"), would the AI indicate *exactly* wings level?
cbl.