I haven't checked recently, but in the past ASDF files have had significant code even before the system definition. It was common to find conditionals based on *features*, comments for users to select from a few customization options, numbers being read, etc.
Once you have read the form, it is too late to modify the environment that is used to read it. Thus I suggested that we adopt a convention to check for a "magic header" that changes the syntax used for the rest of the file. The suggested syntax was meant to make this header invisible to legacy systems, unambiguous in meaning, extensible, and fairly easy to parse for new sysetms.
Emacs already has two such conventions. If someone wants to implement such a thing, that's the way to go. I'm sure Robert accepts patches, though in this case, you might want to start with an ASDF extension, a la asdf-encodings, that does exactly that with respect to encodings (many thanks to Douglas Crosher).
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org If money is your hope for independence you will never have it. The only real security that a man will have in this world is a reserve of knowledge, experience, and ability. — Henry Ford