... This is an interesting one. I think there's no solution to this. The call to A::Init isn't done via the virtual function table (as you know explicitly which one to call) so it falls under non-virtual functions, which you cannot mock.
A complicating factor for adding this is that if you took the address of A::Init on B you'd still get the vtable pointer, so you couldn't even replace the function if you wanted to (say, using the C function mocking logic).
I can only recommend the generic refactoring solutions to solve this - make A and B independant classes, make B contain A (private inheritance) or extract an interface from A and B and turn B into a decorator. The last sounds best but it depends on what A and B actually are...