I use the 600EX-RT with a Better Beamer on the RRS B91-QR with FA-QREX2 on my 600 II. I've been using the OC-E3, but I have a SnapQR cold shoe, and triggering with the ST-E3 would be a little cleaner than the coiled cord - great idea!
Hi Neuro, I bought the Better Beamer specially for use on the 600f/4 II, Haven't actually tried it yet, but I have hopes the Better Beamer can help overcome the 200mm Zoom limitation of the 600EX-RT, the 200mm Zoom seems to work just fine when in use on the 200-400, 300 & 400, but I've tried it with the 600 & there's a lot of light fall off/spread which the better beamer should help with.
I found the ST-E3-RT Triggering via the Camera worked well for me, especially when you need to change the position of the 600/s on the Ring, I imagine that could become a bit of a Tangle using a cord to trigger.
Just an added reminder for anyone wanting to go the RRS way, keep in mind if you want to attach your 600EX-RT to the RRS extender, or any RRS piece of gear that has a dovetail fit, you need a "Snap QR Adaptor", RRS make a number for different fit outs, when I first bought my set up I had everything I needed, except this little bit of gear to actually attach the 600 so it would all work.