Given that Wine uses winelib for its builtin exe's and dll's, and that the way it works is not much different from a PE exe or dll, I don't think winelib is likely to be at fault here. My guess is that you are running into an ordinary Wine bug relating to your specific dll (and probably Wine's 64-bit support, since that's a little-used feature).
I think the best thing you can do is file a Wine bug and simplify your test case as much as you can.
Are there any freeware 64-bit Windows programs that would be able to load your plugin dll? If so, do they work in Wine?
How hard would it be to write a simple program that does just enough to reproduce the crash? If that works, you could recompile it as a PE exe and see if that makes a difference (I don't think it will).
Do you have the source code of the dll that crashes? If not, maybe you should write your own stub plugin that you can compile for 64 bits. That way, you can at least verify that you ported your application correctly, and that it can work for a 64-bit dll that's not affected by Wine bugs.