Didn't makita used to make a hollow mortiser for timber?
I'd make some samples first for the architect, then price out how you'll do it.
Cheaper way is to peg with standard pegs and use square plugs inserted from both sides.
Expensive way is to cut them all by hand.
Perhaps slightly less expensive way is to buy a woodworking mortiser and modify the base and cut from both sides. Clean up by hand.
It seems of you use a draw bore that your blocks will need to be extra long with some taper to them so you can drive them in.
