Any reason I can't use popover batter to make one huge popover without separating into little cups?
You absolutely can do that. Brits call it Yorkshire Pudding, and the technique is actually fairly forgiving. You can make them muffin sized, or you can make one big popover. Just don't mess around, popovers get their "pop" from steam. A medium cast-iron skillet (9 inches or 23 cm, roughly) is perfect, you can do it in a square tin too.

My British buddy says: http://chat.stackexchange.com/transcript/message/25537746#25537746