OK, I've got two issues here.
Both issues are not happening in Native, so I suspect I've stumbled into something rather nasty and hard-coded indeed. Just not quite sure... where, yet, lol.
1. Reflections just aren't rendering, period.
I've tested my shader, and it's not the shader; if I, say, reference ScreenTextureSampler rather than ReflectionTextureSampler... it "works"- I get an output that's sensible, given the input (ScreenTextureSampler is, apparently, the screen, shot from the POV of the camera- how this differs from the ReflectionTextureSampler, I don't know- I always figured the latter was the same as the former, but we just rotated it with a matrix... but I digress).
I've tested getting rid of my shaders entirely, just to make sure I'm not nuts... and that also doesn't work. So, whatever's happened here is 100% not shader-related. But there's no "reflection" material, or anything else that's obvious. Maybe a cubemap somewhere?
Theory: there's a Material somewhere that needs to exist, or a Texture that is actually used as the buffer, rather than this being internal to the engine. Which is nuts, but frankly, I don't have many other options.
2. The skybox shader shouldn't ever fail, but... hmm; I can see one idea to test.
I've got a working theory there; I'll take a look at that and get back to you.