Yes, that is no problem for CgLib. I have even tried to do it and it was a no problem for me. I use cglib-nodep-2.1_3.jar.
If you want to d.boo(); to do nothing I see no problem here.
If you would like to implement something like Java8 default methods there is a little more tricky with passing a state of an object.
The only different is that you need to create an object by Enhancer. If you want to change how "new D();" behaves you should try AspectJ. I think its possible to make it work with AspectJ, but eclipse compliator will always give you problem. its should be possible to ignore compiler errors.