Confirmed: Pi Power & Model B+

Here's a model B+ sitting in my hand. In this picture, I've installed a 14 pin stacking header on the remainder of the GPIO header.

A side view of the above. There is a slight gap between row 13 and 14, but it's not so wide that a connector won't fit.

And to prove that, here's a 26 pin header plugged in across the gap.