In ASDF1 and ASDF2, indeed, .asd files are read from a temporary package ASDF~D that uses ASDF. In ASDF3, we're using a permanent package ASDF-USER instead, and usual hygiene rules apply.
So, if you define your own operation classes, you need to create a new package?
You already needed to, to be able to name, modify or redefine them afterwards.
In practice, everyone already did, at least in Quicklisp.
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org The major advances in civilization are processes that all but wreck the societies in which they occur. — A.N. Whitehead