Dave,
instance_select_query is defined as a method of the meta-class CrClass (CrClass ad_instproc instance_select_query ... in generic.tcl).
Vocabulary: A meta class is to a class the same as a class to an object. The instances of a meta class are classes, the instances of classes are objects. Meta-classes are used to configure classes (sometimes called class factories) and to provide methods for classes.
Since instance_select_query is called on classes (and not on the instances of the classes, e.g. pages), defining an instproc for subclass is not correct (it provides methods for instances), but one can define a class specifc method SubClass ad_proc instance_select_query...
Clear?
