I used cherry for my outwale and sassafras with a black walnut strip for a little more depth, and to reinforce my scarfed joints for the inwale. Didn't have any solid plan in mind, just thought it would look good. I didn't have any ash and I had plenty of cherry. The outwale I made about 11/16 wide and an inch deep, so I could cover part of the strip below my shear strip, I thought it would be more structurally better.
Here's what I was thinking about screwing instead of glueing. This is my opinion: When you drill or punch a hole thru a strip of wood, you are now weakening that piece in that spot. You will be displacing or removing wood to make the hole, and also displacing the wood fibers. With glueing you are adhering that whole piece, intact, un-fractured, un-compramized and now it has become one with your canoe, laminated.
Roy