Ok I didn't get it up and running. So I switch to CXF - DOSGI, using the dependencies listed above. (But different version) Then I had to solve another issue due to a missing http transport. In the end the code for accessing a REST service via an OSGI REST client looks like.
JAXRSClientFactoryBean sf = new JAXRSClientFactoryBean();
sf.setResourceClass(serviceClass);
sf.setAddress("http://" + getServerUrl() + "/cxf/" + serviceName);
sf.setProviders(providers);
ConduitInitiatorManager cim = sf.getBus().getExtension(ConduitInitiatorManager.class);
cim.registerConduitInitiator("http://cxf.apache.org/transports/http",
new HTTPTransportFactory(sf.getBus()));
service = sf.create(serviceClass);
Further reading link