Dear Nathan,
at least as far as quicklisp-included software goes, your ironclad and nibbles seem to be the only user of ASDF's :if-component-dep-fails and :feature features. These features are a crock and I'd like to plainly remove them. In the future, I might provide a better-thought-out replacement that actually works. In the meantime, the backwards-compatible solution would be to use #+sbcl and such; yes it breaks an abstraction barrier — but at least it doesn't require ASDF itself to break its own abstraction barrier internally.
Could you "fix" your .asd's to not use these features I'd like to drop?
Does any other person who cares about ASDF cares about these features?
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org If you make people think they're thinking, they'll love you; but if you really make them think, they'll hate you. — Don Marquis