[eluser]Rob Stefanussen[/eluser]
Paul, thanks for the tips, I was able to get it working! I thought you might like to know about a little discovery I made while I was at it. At first, I was using the naming my YML with "_form", but that wasn't working. After digging around in the code, I found out what was going on. From what I can tell, there's nothing special about "_form" to CodeEx.
You can either do your CRUDs using a custom controller, or use the default controller. Right now, I'm using the default controller until I have a better handle on things. The reason YMLs don't need the form_setup directive is because it's defined in the $config array in the controller. It turns out that the trick is in the naming of the linking table and the foreign keys, which I wasn't able to figure out until I read your reply. I was a bit disappointed that CodeEx relies on an inflexible naming convention for this, but beggars can't be choosers, right?
I was able to get my tables linked up with these YMLs: