The real motor-generator combination you mention would actually run down, because the initial energy would gradually get dissipated as heat generated in the wires, via friction, etc. If we ignore that, I don't see any reason why it would or even could reverse.
The earth's magnetic field does reverse every 150,000 years or so on the average, with very irregular timing. However, it's a much more complicated system, more like the swirling patterns formed in a pot of water on the stove than like a few organized coils. Part of the reversal process involves the current patterns breaking up, changing directions gradually, etc. The motors you described don't have big streams of currents that can break up, or any continuous range of directions to drift around gradually.
Mike W.
(published on 05/16/2011)