In the past, I read a book about installing fence posts. In there the author strongly cautioned about encasing wood in concrete, especially in an "open air" environment.
The reason being that, in the example of a fence post, encased in a concrete "cup", this cup would/could fill up with water and then the fence post would be sitting in a cup of water. Water creates rot. He suggested if you wanted to use concrete in your fence post installation process to just make it a cap over the fence post hole. Any water hitting the fence post that would run down the fence post and in between the fence post and the concrete cap would then run out the bottom into a bed of crushed stone, and hopefully not rot the bottom of the post.
Now, here, in this application, is there a possibility of rain water getting into the concrete mortise created if you were to use Shaun's idea?
It's an interesting idea but unless you can be sure that no water would run into this concrete cup created by the spline, it might not be the best solution.
Also there is the possibility of the wood wicking up moisture from the concrete. There was another discussion about what to put between the post bottoms and concrete to prevent this. I'm not sure if it was here at this forum or another timber framing forum. But I've seen barriers put between wood and concrete to prevent moisture from wicking into the wood. Encasing a spline tenon in concrete would have to be treated or prepared so that the moisture from the concrete didn't wick into the wood spline.
And what about the mentioned “hold down” issue?
Some more careful thought or other ideas may need to be researched. Good luck, Jim