Yeah, we'll probably end up going this way, but I'd like to do some due diligence here. ddraw caps are an incredibly fragile thing, with applications making stupid assumptions.
There are some GL games that query GPU capabilities via ddraw. I came across at least one ancient game that broke if certain properties mismatched between ddraw and d3d8. (Though I think in both cases games were more interested in PCI IDs and video memory size than minutiae texture stuff)
I'll see if one of my Windows boxes supports large enough textures to check how Windows handles the situation. It's possible that the game is just broken on Windows, or held together by some compatibility shim.