Thanks for all the great ideas. I like the tusk tenon, though assembly might be an issue unless I cut a slope into the top of the mortise so it could be inserted at an angle...maybe that's a solution?

As of now I'm leaning toward drop-in joists, and spending some extra time on the flashing. Dave, it's good to hear that weep holes have been used historically, though I hadn't thought of them filling up with insects, etc over time. What do you all think about using a gouge to make a sloped channel at the bottom of the pocket? Like this: