After a bit of struggling with calling KIO slaves from Qt Creator, I now have some basic integration with the Open Collaboration Services going. My friends from opendesktop are shown on the KDE Creator welcome page.
This in itself isn't really useful yet, but it's a first proof of concept for the integration of online services in the KDE SDK. Integrating access to the app listing on kde-apps.org would be one of the next steps.
The trick to use KIO slaves in Qt Creator (or any other Qt program, which doesn't use a KApplication), is to instantiate a KComponentData object. This provides all what's needed to run many of the classes which rely on some global KDE specific data being available.