In general I think the rectangular tuns are more suited to batch sparging. The shallow grain bed makes it easier to fully stir the mash during sparging. However, it is my opinion that large flat bottom MLTs lend a larger perceived dead space. I call it perceived dead space because when testing it without grain, it would probably drain pretty well. When you add the grain, it stops a lot of liquid from flowing to your outlet along the flat bottom. It's not quite grain absorbtion nor deadspace, but it is wort lost nonetheless. Of course, this effect is negated if you use a full false bottom so I think that rice cooker would be pretty sweet.
Dumpster? You've got to think outside the box when you want to scale up without dropping money on some retired dairy tanks.