I haven't gotten this to work. I wanted to have a small trigger on my Leica M240 to control multiple 600EXRT flashes with manual output.
The closest I've gotten is to use a 600EXRT on the camera set as master and using the optical trigger system instead of the radio system. Unfortunately it means one flash has to be on the camera. Alternatively, it might be possible to trigger the master flash using a radio slave on the camera with a receiver on the flash. The master will then trigger the other flashes. But I have not tried this and don't know how bad the latency will be. In this respect, when I used the 600 as master on the Leica, I could not use 1/180 maximum flash sync speed, but 1/125 was OK.
Hope this helps somewhat...