Until then, any Canon EX flash can get the RT treatment with the Yongnuo YNE3-RX Wireless Flash Receiver.
I got good results with an older 580EXII and with a 430EXII.
They can be controlled from the Canon ST-E3-RT or a 600EXRT master and so bring older flashes to the same system. Also, they are relatively cheap. Mine prove to be reliable, too. I also use them for studio flashes (only for triggering, not for power adjustment, of course), so I can ring my 600EXs and my studio flashes to the same wireless system. Cool thing.