Hi Arvid,
Try building the model locally, i.e. run Simulink.BlockDiagram.buildRapidAcceleratorTarget(modelname); on your "host" MATLAB session before calling parsim. You will notice that the build process generates a Simulink Cache file (slxc). If the architecture of the parallel workers is the same as that of the host (which is always true if you are using a 'local' pool of workers). parsim will send that slxc file over to the workers and even though the SetupFcn will call the rebuild, it will just skip the actual build. If you parallel worker is a different architecture then you will have to generate the slxc file for that architecture and copy it over to host machine.
Please try it out and let me know if you run into any issues.