> To do that we need to modify [1] the VM to support instance 
> base behaviour in an efficient way.

Use uniclasses (the way eToys work) - they give you the exact same behavior
at a little cost in terms of space and it is as efficient as it gets. You
don't need to hack the VM for it at all.

> [1] Any VM hacker can estimate the effort to modify the VM in 
> this way?

Zero ;-)

Cheers,
  - Andreas