Yes, they can, Steve. In Central and Eastern Mexico, it is not at all uncommon to encounter a tamal de X (pescado, hongos, etc.) that has been either cooked on the comal or poached in a shallow liquid. Most commonly found wrapped in hojas de maiz, but sometimes in hojas de plátano (which are especially aromatic). I had no idea that such tamales existed until I ran across them in a couple of (very) rural markets and at a puesto in the plaza in Tlaxcala. Guadalupe Pérez San Vicente, in her Repertorio de tamales (Cocina Indigena y Popular #15) states that her research led her to the conclusion that the term "tamalli" referred to something that had been "envuelto cuidadoso" - or, carefully wrapped. True, by far the most common in Mexico, and certainly in the US, are the tamales that are steamed in a vaporera.Finally the warm fish packages are passed around and it's a pretty nice moment. Everything is cooked to perfection and two seems about the right number per serving. There were no leftovers, not even of the soup.
I keep experimenting with this concept and it keeps getting better. It started by my seeing a clay pot of these fish tamales in both in Morelia, Mexico. I've attempted to recreate it before. Every time it improves. You start with a broth. This was half bean broth and and half water. The zucchini were sliced very thin and added with onion, parsley and celery. This continues to cook until you have an acceptable broth. Soaked corn husks for tamales were soaked in warm water for a couple of hours to make them pliable. In each package I added an hoja santa leaf, two pieces of cod, morel mushrooms, zucchini slices and a mix of thinly sliced onion and jalapeño that had been marinating in olive oil. This was tied up with string to make a little package. This is what I had on hand and sounded good. I would encourage you to do the same. Got shrimp? Use it! Some other vegetable? Just slice it thin enough to cook in the relatively short time you have before the fish overcooks. They cooked in the broth for about 20 minutes. While the fish gently poached, I fried up some padron peppers in duck fat. Oink. I mean, quack. This salad of cucumber, yogurt, lemon, garlic and mint continues to be a summer favorite but my fickle family is getting tired of it. During these hot summer nights, I think it's the best salad ever. I cooked the fish in a red clay cazuela from San Marcos Tlapatzola in Oaxaca. It's become a dedicated tamal de pescado pot. Inspired by a traditional barbacoa in Hidalgo, I decided to add garbanzos to the bottom of every soup bowl and ladle the hot cooking broth over them as a soup course. If all goes as planned, the tamales flavor the broth. Even the corn husk adds a little something. I was confused what to call these. Tamales doesn't seem a hundred percent right since there was no masa but my very clever friend Sharon Peters responded to my query if they can be called tamales: