Could be, Brian. Although seems like we need the range of motion witnessed in the knee flexion of the GHR; in the GHD situp there's no major movement around the hip (because it doesn't bend that way, of course). Mainly just isometric. Garrett's example might be closer.
Garrett: so in your view, the radical difference in efficacy between those two movements (assuming there is one) is the moving-body vs. moving-extremity distinction? That's the difference that makes the difference?