Jews have used various types of earthenware ovens throughout the centuries. Unfortunately, most of these ovens do not translate into English or other languages very well. Also, most of these oven types were lost to Ashkenazim for many centuries as the type of dirt, and the weather required to have such an oven made it nearly impossible to make them.
Here is a list of all the types of earthenware ovens in the Talmud, please specify which kind you are trying to make so we can help you better.
You are correct, one cannot kasher an earthenware k'li. The only exception is that if you heat it to a temperature of a kiln, which is the point that it typically melts and becomes new, and in that case, you will destroy your oven. But in general, only the surfaces that actually touch the food become a k'li, and therefore only those spots absorb taste. Therefore, depending on how you construct your oven, you can swap out sections of it to cook different things.
Here is a picture of my earthenware oven, which is most closely related to the תנור oven mentioned in the Talmud. It was built out of flowerpots from my local hardware store, and the top most earthenware piece/pot/top can be removed and replaced. So i have one "top" that is parve and chametz free, and one "top" that is dairy and contains chametz. i use this oven to bake normal flatbread and grilled vegetables during the year, and switch out to a different "top" to bake matzah during Pesach.
As for sources, any sources you have regarding earthenware apply here. The surface (where food touches) of the oven becomes a k'li. So as long as the food doesn't touch a specific surface of the oven, that section of the oven does not become a k'li.
The only concern might be with vapor/steam. If the food is very liquidy, then there is concern that the steam of the food might absorb into the earthenware. If the food is more solid, then there is less concern regarding vapor/steam. In the end it's up to you to decide how strict you want to be regarding this issue, as there are very many opinions regarding this issue. Here is a document that has many relevant opinions regarding this matter.