This one took a while to track down, so I thought I would write down our solution.  It turned out to be easy.  We simply added an interface to our custom content type's class as follows:

First import the necesary modules:

from zope.interface import implements
from plone.portlets.interfaces import ILocalPortletAssignable

Then we added an implements line to our class:

class SiteFolderCT(BaseFolder):
    """An Archetype for an SiteFolder application"""
    implements(ILocalPortletAssignable)
    <the rest of the class code...>

I hope this helps out others.