Overloading DoPropExchange()

I wish to store to disk certain properties associated with each group text item in a task panel (it is a text string comprising a database key used to look up associated information, but that should not matter).

Since serialization is handled through DoPropExchange() I assume I must overload some part of that mechanism.

If I declare a class derived from CXTPTaskPanelGroupItem, then overload DoPropExchange() in that class, I am faced with the problem that insertion of my items into my group is done through CXTPTaskPanelGroup::AddTextItem(), which returns an object of type CXTPTaskPanelGroupItem *, which does not support my overload.

My first attempt was to derive a class from CXTPTaskPanelGroup, and have it create and insert my item. But the problem remains: the stored item is not of my class, but the base class, and the serialization/deserialization code I insert into the derived class will not be called.

What am I doing wrong? Remember, the objective is to store a small invisible bit of text along with a task panel group item, and to read/write it out of/into the same archive as is routinely done with DoPropExchange(). This must be easy.

Thanks

Eric