I haven't read the entire thread, so apologies if this has been suggested, but canned tripe is irresistible to hounds here, even those a bit under the weather. And being as . . . aromatic . . . as it is, you don't need to add a lot to their meal to make it appetizing. Keeps well, well covered in the fridge, and does not need to be heated to be appealing, which is a very good thing : )
We get Tripett, but there may be other brands.