It seems that I was right and wrong. The instep is generally considered to the the bottom, inside, curved portion of the foot. But I have not yet found a satisfactory term for the top middle portion of the foot. "Midfoot" and the ever-obvious "top of the foot" (as opposed to "the toes", I suppose) seemed to the the simplest terms I could find the for area above the 5 metatarsal bones.
As a writer, I like to have the right word in the right spot. (Of course, writing or not, I just like to be right. But you might have guessed that already, right?) As an artist, I found a couple good resources for those interested in drawing feet, or understanding them from a visual accuracy perspective.
The first is The Foot Thread on WetCanvas, where a series of studies and exercises lead you through different and relevant portions of the foot. It's fascinating even if you don't plan to draw detailed feet.
The second is a Wikipedia article which may drown you in Latin, but has some great illustrations of the various muscle groups that influence the foot and lower leg. What a mechanical marvel!
So have a look and then step out and put your best foot forward.