my "XML Director" connector content-types does not show up as default in the navigation (portal_tabs).
Is this due to the fact that the content-types is not folderish (derived from Item)?
The content-types does not use the "Exclude from Navigation" behavior....so how can you make this type available as portal tab?
Choose your weapon:
a) own indexer for exclude_from_nav that returns always False
b) behavior with exclude_from_nav defined to return False
c) subscriber on object added, which sets an attribute exclude_from_nav to False
Like @jensens found out, you need the Exclude from Navigation behavior enabled, otherwise the content type isn't shown. IMO, that's a bug which should be fixed.
So - rephrasing - use this behavior or go with @jensens option a).