Hi is it possible to remotely trigger an EOS 70D using a 600EX-RT? For example, I'd like the EOS onboard flash to be the slave and the 600EX-RT to be the master.
No, you have to have another RT unit on the camera hotshoe to use the remote shutter release capability of Canon's 2012-and-later cameras. The built-in flash cannot work for this, as it has no RF capability--only optical (i.e., it blinks out light signals, but can't blip out/receive radio signals).
It might be easier to just get cheap radio triggers to be your shutter release, or a Yongnuo YN-E3-RT/YN-600EX-RT.