OK Daniel,
I looked at the neck again and your are right on target. There is addition wood in the heel to accommodate the carving of the tenon. So, adding a piece would not really work as the heel would end up too deep and out of proportion.
So, I am considering my first carving, after some practice on scrap. I probably will do a straight tenon rather than dovetail just to keep my nerves in check. I am not a woodcarver at all, so this is going to be a significant challenge as it is! I have a friend who builds hollow body archtops with straight tenons and I think I can handle that as a first experience.
Now to decide how deep the tenon should be. The neck block isn't too deep, so I assume I shouldn't go into it more than 60% or so. Make sense?