Accepting we were playing the on the original ogre board with the stacking limit of 1 unit per hex and the ogre is always only considered in the front hex, does that mean an a unit can occupy the rear hex of the ogre model with no penalty due to it being an empty hex?
As others have stated already, the short answer is yes. The rule that applies is 3.04
3.04 Most Ogre counters are two hexes long. Always treat the Ogre as occupying only the front hex of its counter or miniature.
So regardless of the physical size of the counter, the Ogre is in only one hex. The Ogres are big, but they aren't that big :-)