.... and action, to me, generally suggests something either horizontal or vertical (not necessarily strongly so, but certainly not square!)
Huh, I feel that both vertical and horizontal are fairly static. "Action" to me means diagonals! It is not difficult to fit an action shot in a square; instead of very upright poses, think running, leaping, holding a bow/sword out away from the body... those kinds of things.
I'm having trouble keeping my action shot from running outside its quarter page.
I hope you guys aren't being so strict about the 1/2, page, 1/4 page thing. I said "approximately." Look at Wendy's pin-ups, the size relations are fairly flexible. On most of them, the portrait takes up more room than 1/4 page and the action shot takes less (Timmain and Skywise are reversed though).
So don't divide your page so inflexibly. I am finding that having the "sexy pose" not be such a rectangular 1/2 page does wonders for the composition. If it goes over the 1/2 mark on either top or bottom and less than 1/2 page on the other end, that leaves room for the portrait where more space is left and the action shot where less space is left. Since you can have a bit of scenery in the "sexy pose" it is fairly easy to make it non-rectangular.
Of course, it does change the rules a bit when your character can fly.
My Aurek piece has an atypical layout for these pin-ups, even though all the elements are pretty much the same proportions as Wendy's.
I'm afraid two out of three of them are somewhat less than truly action-oriented.
Well, look at Wendy's. That is about the same ratio she came up with, too!
Warriors need
real action poses! Gardeners and healers... not so much.