Hey Joe,
I am by no means an expert, there are tons of people who have been playing in the mud longer than I've been alive in some cases, but what I lack in years of experience I try and make up for in research.
The thermal mass of cob depends on a lot of factors, such as clay type, ratios of sand clay and straw, as well as what it is coated in. As I'm sure you are aware, thermal mass is basically the objects ability to retain heat energy. So since there really isn't one exact science to cob building, there are a lot of factors in your design that kind of depend on the environment you are building in.
That said, here is a video I found last year in researching earthen floors with radiant heating . I wish it wasn't so cheesey, but if you can sift through that, you might be able to find a little inspiration worth the effort, this guy really did a decent job from what I can tell.
https://www.youtube.com/watch?v=15oN2MFjdVU
Can't wait to see pictures of your finished project!