After revisiting the code at eggheadcafe, you will note that is writes the class on the fly and then compiles it and executes the function EvalCode The wrapper itself would necessarily have to be the evaluator. You could netload the evaluator and send it the string to evaluate, but it won't help you in the IDE. Presumably you want to execute .Net functions in lisp (as evidenced by your function above).
Why build your function around the code provided, netload it, and pass to it the string needed to be evaluated. Presumably the arbitrary class would be self depreciating when the class is disposed, thus unloading it wouldn't be necessary. It would require some testing to be sure, but based on what I've seen, it could be really cool to introduce some .net functionality to lisp.
When I get time, I think I will look into this a bit more.