Frank -

I assume that is so because the wood fibers run in different directions in a ply? (as opposed to the same direction in a normal piece of wood.)

I'd considered using plywood in this manner, but thought I might get a stiffer vertical with the glass 'ribs' like in the graphic.

So - I could do the beam then with 14-16 'slices' of 1/2' ply, bonded together with a couple layers of 6 oz cloth between each, make it 1 3/4" thick in the vertical dimension, and cap it with 1/4" of glass...? All this would get the same final treatment - completely encapsulated and bonded to the underside of the deck, as noted above.

It might be quicker to do that way. One thing I liked about all-composite, though, was no worries about water penetration. Could take care of that with much care in sealing things up I guess...