Er.. A ZClass based product doesn't have a convenient __init__.py that I can tweak, nor is there anything in the zclass management interface that I can use, either. What have I missed here?
I just tried this... You don't have to do anything magical with ZClasses to have them show up in the base class list during the ZClass "add" screen. I suspect that your problem is due to a bug that (I think) has been fixed in CVS. There is a bug in <= 2.1.6 ZClass add UI that prevents all the available base classes from being listed until you hit the "right arrow" to move one of the base classes into the base class list. Once you move one base class into the list, the addable base class list will be updated with all available base classes, including (hopefully) your other ZClasses. This is a silly bug.