Are we thinking about any part of d3dx in particular? We may want to do different things for different functionality.
My guess is, that's probably where we'll end up. Agreed, math functions should be the easiest to share and most useful for vkd3d as well. Similarly for "shader" stuff, as you mentioned earlier.
WRT texture functions, that's problematic at the moment mostly because of the Windows dependencies (mainly WIC). That's especially true for the current d3dx10 implementation and, in fact, one of my primary immediate motivations for sharing is to reuse (most of) the d3dx9 implementation, which is quite a bit more self-contained, elsewhere. Once that is done we could start to investigate porting that part to vkd3d and e.g. initially supporting non-DDS file formats only on Windows.