I think you will find most of the exotics tend to be dense and brittle. (...)
That's a false statement. You're talking about tropical woods. An "exotic wood" is simply any wood that is not native to your area. In my case, osage is an exotic wood, as are red oak, hard maple and black locust. I live in Europe, and those woods don't grow here naturally and are hence exotic.
I do, however, pretty much agree with your statement that most tropical woods are better suited for bellies than for backings. Most of the imported tropical woods are indeed dense and brittle, but that's just the choice we make on what to import. There's thousands of exotic trees, and many are weak, soft and light. Indigenous people often make self bows out of local tropical woods, which implies good tensile strength of the wood. But these woods are often simply not imported. However, ipé does make a good selfbow (as is demonstrated by Surinamese and Brazilian people), but the typical boards we can buy have their grain (severely) violated and are hard to read, thus require a backing.
Tropical woods like wengé, zebrawood or rosewood are better suited as a belly. Their tensile strength simply isn't great. But the tensile strength of walnut is pretty good. I thick that walnut backed zebrawood/rosewood could actually make a pretty decent bow! Yes, that's the woods the other way around. But it'll still give you what you're after, right? I personally prefer a triple laminate for such tropical woods, as it allows the bowyer to use even three different woods. Hickory backing, zebrawood core, and bubinga belly would be a stunning and very good combination
