document why the subclassing is needed/what it does.

i.e. "Subclass of XYZ to avoid validation fails when property expressions are used in connector description - this class only works when {@link classthatdoesthereplacement} is used when loading the connector descriptor" 

Thats at least how it looks to me - that the property replacement support is "spread" over two classes instead of just done in one place as one would expect?