[jgw]

Bit late replying to this, so I hope you see it, but as I noted elsewhere, the key here is something called a "reader macro". This is a mechanism by which you can control the behaviour of the Lisp reader (i.e., the READ function), and occurs before regular macroexpansion. I use [] to denote s-expressions in the embedded-language domain, and {} to introduce native syntax, which passes its contents through to an actual parser - which returns s-expressions. I can print the resulting forms in native syntax, embedded-s-expressions, or plain Lisp. It's a neat demonstration of the superficiality of syntax.

Perhaps another interesting observation is that what I'm building today is not actually what I originally set out to do, which was more limited in scope. Perhaps another weakness of CL is its tendency to seduce you into feature-creepism. :)