"multi morphic" classes

[Russell Lewis]

I apologize if I'm missing some subtlety, but it seems to me that this 
would be pretty straightforward...

How about just building a class by hand using a struct?  You declare  a 
struct type which contains a bunch of function pointers, and each 
instance struct has a pointer to that common vtbl.  Then you write 
wrappers as member functions of the struct, which call the vtbl 
functions, pass "this" as the first argument.