[vlehto]

>>Each car is designed around the battery since it's such a large piece of the car.

This is not exactly true. Battery is heavy piece, so it has to be low for low center of gravity. The bottom mount is going to happen anyhow. Tesla and Leaf both have their battery in the bottom.

The only real problem would be that you can't use the battery as structural element if it's easily detached. But the bottom center point is not exactly critical for structural integrity, so the result would be slightly increased weight.