I think this is a great idea, and should be a standard feature for anything that calls itself a 'reprogrammable programming language'. After all, the ultimate in reprogramability is being able to recode the interpreter itself.
I have a different suggestion for implementing it: instead of redefining the interpreter, have a macro that is called on every expression before it's interpreted. The macro would compile the expression down to canonical Arc, which could then be executed by the standard interpreter. Not sure what the pros/cons are of doing it that way, but it sounds to me like an easier hack than redefining the Scheme function.
