I suspect this isn't possible in a compiled language like D, but I wonder anyway, has anyone considered partial classes in D? Are they technically possible? How would they work? Would it depend on link-time code generation? I can imagine an implementation where each 'part' occupies a separate allocation, and some pointer in Object or somewhere like that (somewhere near the typeinfo pointer) binds them together.
I often find myself wanting to add some extra pizazz to classes using template/mixin magic, but I may not have control over the original code where it is defined... C# at least can do this, and it's very useful in many situations...
