Loose tenons +1.
BTW, I am so jealous of Al B Thayer's ability to make 6 chairs and still not be barking at the moon! The little tool in the left side of his pic makes quick work of routed mortises.
However, back to your Q. If you bore the mortises as deep as you would have made the tenons, plus the depth created by the angle and 1/16" for glue pocket you'll be good to go. Use a waterproof glue; type III or Gorilla, etc.
I love loose tenons!
Tip: Bore the mortise before cutting the angle!
Last edited by bladeburner; 09-01-2013 at 09:33 AM.
Reason: added tip