The question was (see the original post),  why does a command work in a terminal but not in a method. And, as you know, the answer lies in the scoping. I would say, he (@David Hockenbroch) tries to learn and understand the nature of indirection and is not working on a production grade problem.

But maybe I'm wrong... who knows.

He (@David Hockenbroch) is playing with inderection, using $classmethod() instead of ##class(classname).methodname(...)  does not solve the scoping problem:

ClassMethod testvalidator(class As %String, value As %String) As %Status
{  
  set validator = "sc = $classmethod(class, ""IsValid"", value)"
   write validator,!
   set @validator
   write sc,!
   quit sc
}

The above method gives you the same <UNDEF> error because of non global scoping! By using indirection both variables (validator and sc) must have global scope.

As @Sergei.Shutov pointed out, you can switch off the procdere block by a keyword for the whole class. Additionaly, you can switch on or off the procedure block keyword for a particular  method too. In your case:

class Some.Class  Extends %RegisteredObject
{
/// a procedure block method
ClassMethod ProcBlock()
{
}

/// a nonprocedurblock method
ClassMethod NoProcBlock() [ ProcedureBlock = 0 ]
{
// Caution: All variables have a global scope, hence, they will overwrite variables with the same name, which were created previously. To avoid this, use the NEW command, to protect them (if desired).
}

}