"multi morphic" classes

[BCS]

I'm working on a compiler and have run into a bit of a design challenge. 
The issue is that I want to be able to manipulate a number of aspects of 
the compiler at run time so that command line switches can turn on and 
off a number of different features. For example, modified lookup rules 
or auto typing of function returns.

This is somewhat complicated by the fact that I want to have the bulk of 
the code be unaware of the features that can be turned on. This is 
because I want the default behavior to be to function as a standard 
compiler and the the feature to implement extensions. However I want the 
standard portions of the code base to, as much as practical, be 
independent of the extensions.

One way I have thought of to do this would be to implement the AST node 
types using the default behavior but at runtime construct the AST with 
nodes of types that are modified copies of the base types. This would be 
done by having each extensions be implemented as a derived types and 
then, at runtime, construct new v-tbls that have selected methods 
replaced with the derived type methods. Then this would be used in the 
construction of nodes.

This would give a sort of "dynamically created" type. I think I could 
hack together a (highly non portable) implementation but would rather be 
able to do it within the bounds of the language. I have no idea what the 
syntax for this would be though.

thoughts ideas?