Eric from the colonies here...
how about this idea.
your two curvitational underseat rails that butt onto the curved back lamination, they are, I agreee where the majority of stress is gonna occur, so I'd draw upon the concept of the "clenched nail" or the wedged tenon, and suggest that you place a block between the two rails, perhaps substantial, as in 8.4 material, and temporarily attach the seat assembly to the back. Then, judiciously cut 3/8 x3/8 moritices through the back into the seat rails, perhaps add a few in a curvilinear orientation into the 8/4 block.
then, taper the top and bottom sides of the hole with a 3.8 chisel, perhaps to dovetail angles, ..yer getting set up for wedged tenons here.
You glue the block between the rails to the back, not clearly strong enuf, cause it would just lift off the top ply under stress, but every little bit helps.
You could always use dowels on the ends of this block to attach it to the side rails if you wanted, or just pocket screws.
anyway, you mill up some 3/8x3/8 stock and glue it into the side rails, trim it off just a tad longer than yer 12mm back thickness, and cut a horizontal slot for the wedge.
you put the back in place over the tenons you've created, clamp it up, slather some glue on some wedges and drive em home, creating captured dovetails. Not easily removed or fixed, unfortuanately.
Any stress would concievably be carried throughout the entire thickness of the back, as opposed to just off the front frace.
If the 8/4 block was dowelled into the side rails, and the same wedged tenons were used along it's length, you could likely even triple or quadruple the strenght of the jointery.
OTOH, you could just use wood screws and plugs and metal dowels to achieve the same strength
This is sheer conjecture on my part, it would be a simple task to do a mock up and test it to assess the strength.
I betcha that Rennie Mackintosh fella did the same with his chairs. That high back spells substantial leverage to me.
Eric
in Calgary
we is still just using saddles, eh!
