Take this with a grain of salt, I was under the impression there is a 2nd curtain setting, but only with optical, not with the radio portion. With radio you have high-speed sync.
I was quickly watching a tutorial video today and I thought that was how it was explained. I'm in the market for a speedlite and will decide which one to buy soon.
I to will be coming back to see what Canon says about this.
I have done some searches and although I have tried most everything myself I never tried second curtian sync, off camera, optical wireless with my 580's which I sold. So far I have read that optical wireless, off camera, second curtain sync is not possible wiht Canon is not possible. Something do the instruction order Canon has in their programming. I don't know if this applies to all Canon bodies but based on what I read I think it does. It appears Canon applied the same prgramming to RT.
Here's my take...
Firstly there is no second curtain sync with either pulsed light or radio controlled off-camera flashes in the Canon system if you only use Canon kit.
I know for sure that Pocket Wizard can do it with the Flex / Mini devices added to the flashes. Maybe other 3rd party radio triggers too
In the pulsed light system the issue is that the command to "fire now" would need to be sent by pulsed light while the shutter is open, giving ghosting or potentially additional lighting to the scene. In the case of bulb mode for sure the camera won't know how long the shutter might be open before the flash is needed.
Over radio it should be possible, since the radio signal is not visible to the camera, and it's my request for Canon to add it to the radio system using 600EX-RT and ST-E3-RT via firmware update to flash or cameras.
Brian / p4pictures
I quizzed Canon on this some time ago, and received a standard 'technical issue' response. I didn't push them on it, because we both knew that that official line was in fact bunkum. The SCS function can be achieved with a software, firmware or hardware solution - not trivial, but definitely straightforward.
Move on to a Canon evening at the Hollywood CPS facility some months back. There was a tech guy there who shall remain nameless, who revealed that it is almost certainly an oversight - a legacy from the optical system that the product development team neglected to include in the radio system.
Now, it may not be true, but this guy was close to the facts, and did seem rather embarrassed about it.