So I came up with this. It's got a cross piece horizontally through the end of the through tenon. That cross piece is relieved in the back so that wedges can be put in to pull the joint tight. I did a quick prototype last night and Lori likes it. I think the madrone cross bar with walnut wedges looks nice. If it turns out ugly or fiddly, I can always just cut a single horizontal pin and do it that way.
|Through Tenon, Pinned and Wedged - this is just a mockup using some blocks|