Running the battery to 30% SOC (0% usefull) and then trying to climb a sustained incline at highway speeds seems like it may be an issue. It's my impression that the ICE alone will not have enough generating capacity to power the car without the battery helping. This is fine if only short bursts of power are needed. But, on a sustained climb this would seem to be an issue.
Perhaps the car's navigation system could include elevation data so that intelligent decisions could be made about when to engage the ICE. If you tell the car you're going on a 100 mile drive (by entering your destination into the nav system) then the car could see where along the route you will be increasing elevation. Based on that the ICE could be used to bring the battery to a higher SOC prior to starting the climb.
This should not offset the plug-in benefits on the car, even though the ICE may run at times when the battery is above 30% SOC. All we really care about is that the battery is "empty" when reaching the desitnation, so that plug-in power can be used to recharge it.