I think your best bet is to fiberglass the entire outside of the hull, and maybe tape the inside as well. My Pygmy has fiberglass over the entire exterior, over most of the inside of the hull, and taped joints on the underside -- and with footbraces, back strap, butt pad, and a whole gallon-and-a-half of epoxy, it weighs only 37 lb! That's without bulkheads or hatches. You shouldn't have much of a problem keeping your boat under 45# even if you over-do it with fiberglass. Plus your boat will still be much lighter than straight fiberglass or plastic kayaks.

Another consideration: Only taping the outside seams will mean more sanding, and feathering the edges of the tape may lead to a concave look to the panel. If you fiberglass the entire outer hull, the glass helps smooth out the surface, cover over any small irregularities, and is surprisingly easy to apply.

Good luck! Dean