If hardness and density alone were enough, I’m convinced exotic hardwoods would never have risen to their present ascendancy in woodwind construction. Stringed instruments, drums, and dry-blown instruments like the uilleann pipes don’t have the added burden of breath moisture to directly affect them. The salient point about exotic hardwoods for flutes or whistles is actually not the looks, or only the density, but the oily, highly resinous character of those woods, which makes them comparatively highly resistant to moisture absorption and desorption, and for a breath-operated instrument like a flute or whistle that stability is a huge, huge plus. That’s why ebony never lasted long as a flutewood of choice: it’s handsome, sure, but it’s not resinous, and ebony flutes were notorious for cracking. With uilleann pipes, ebony doesn’t present that problem because the UPs are dry-blown.
As you say, in the northern climes there are functionally suitable woods for woodwinds: among flutes, most notably box, and for recorders, traditionally fruitwoods such as pear. But as with almost all northern hardwoods, they aren’t resinous, and this is key. I can’t speak to others’ practical experiences (beyond simple issues of tone, etc.) with those woods, but I can tell you of my own experience with dogwood, a highly suitable tonewood for flutes. Dogwood is hard as nails - I dropped my dogwood flute endcap-first onto a tile floor and it suffered not so much as a nick! - and it’s so dense as to hardly show any grain pattern at all. Sounds perfect, doesn’t it. Tone and response-wise, it is. BUT: the wood not being resinous, that flute absorbed moisture so fast and so much that I had to wrap the tenons to fit wobbly-loose in the sockets because of the swelling to come, otherwise at the end of playing I couldn’t get the instrument apart, and I had a few scary moments with that at the first. Mind you, this was still despite the flute having been pressure-treated with linseed oil, and me oiling it every day. It drank up oil like a fish, but still it absorbed and desorbed moisture like nobody’s business, and this also showed in subtle, temporary changes to the embouchure cut’s profile if you looked closely. Also note that for all the talk of oiling one’s blackwood flutes, really the blackwood doesn’t need it in the long term; it just needs to be kept properly humidified. Thanks to its highly resinous character it absorbs and desorbs so slowly that you wouldn’t notice it. Totally different story with the dogwood: in comparison, that was like wild mood swings right in front of your eyes. It was for that alone that I was never quite happy with the instrument.
If non-resinous hardwoods are to be used for serious working flutes and whistles, to me it’s an article of faith that you have to address the issue of absorption, and that means paraffin impregnation or something on the order of lacquer coating. You can’t get away from it if you want stability.
I really don’t think oaks would suit transverse flutes because, for all their strength, oak grain is coarse. But I’m thinking in terms of the embouchure cut, and just guessing as to whether that would make a difference. I’m pretty sure maple, particularly rock maple, would be better. Another drawback of dogwood is sourcing: it’s extremely slow-growing; the trees are small, so optimal pieces have to be sought, and there’s not a lot of it around relative to its growth rate, so it could become endangered, too, if clamored after. If naturally resinous wood were all that were needed, certain pines would do, but of course they’re too soft and unsuitable for serious instruments.
In sum, exotic hardwoods are not the prestige choice just for their looks.
