I don't know about the others, but I don't like the idea of using a battery car when switching is my goal. The whole purpose of switching is to leave/pick up cars, not drag one around the whole time.
But, why not use a caboose as a battery car? A local switcher with a caboose (aka rolling office) would be a normal thing I would think, where else would the conductor do his paperwork?
The 2000Mah battery pack I use is approx. 1x1x4 inches, may fit inside a caboose if there's no interior, or if you could find/make a pack that would fit between the sides of the molded interior you could keep the interior. A 1000Mah battery is even smaller and may be a better choice if going with a caboose/battery car. A 4-cell inline battery pack may fit nicely.
But then you have a tether between the engine and battery car.