Actually the rules for Flight, Leap and Incorporeal suggest that pretty strongly when they say the model ignores terrain while moving. The FAQ does say to count the vertical distance you've moved to stop you getting "free" movement when going between levels. It doesn't say anything about counting the Ht of obstacles along the route and if it did that would make the above rules pretty useless.
If a model wants to leap over a Ht 5 building that's 4 inches wide and land on a Ht 1 cart on the other side that's fine. It needs 4" of movement plus one for the vertical (per the FAQ) and whatever its base takes up. Gremlin Guy's scenario is the exact same but upside down, and I don't see why that would make any difference.