*default-character-element-type* merely provides the default behavior. If enough information is supplied, then a string of suitable type is constructed. For instance, the form:
(make-string 3 :initial-element #\Ideographic-Space)
constructs a string of a type that can hold its elements, regardless of the value of *default-character-element-type*.
LispWorks User Guide and Reference Manual - 21 Dec 2011