Instead of using .def file just insert pragma comment like this
#pragma comment(linker, "/EXPORT:SomeFunction=_SomeFunction@@@23mangledstuff#@@@@")
Edit: Or even easier: Inside the body of the function use
#pragma comment(linker, "/EXPORT:" __FUNCTION__"=" __FUNCDNAME__)
. . . if you have troubles finding the decorated function name. This last pragma can be further reduced with a simple macro definition.