Generic copy function
Given the restrictions you outline that seems sensible, I also couldn't
find a CDR for it (in contrast to an extensible equality operator for
example), however I'd probably try to find a better name than UCOPY.
Also, since you often might have to choose between shallow and deep
copies, perhaps introduce a parameter for that.

The method for STRUCTURE-OBJECT isn't guaranteed to work, so it might
be more advisable to leave out that method and require the user to
implement them by hand.

Rest looks fine to me.