I often use Av mode for flash stuff for candids, etc, because it does meter for the ambient. I'll typically set the meter to -1 and then adjust FEC accordingly. Set the ISO manually high enough so that the ambient is about 1/100-1/200. Enable HSS on the flash in case you walk into a brighter area and the shutter speed goes past the sync speed of the camera.
It might be worth checking out the manual to see if you can override the max iso of 400 constraint. Is it only with flash attached, etc? Perhaps there is a setting to remove it.
Will HSS be required if the shooter walks into brighter area? The "bright" ambient light will take care of the black bars when shutter speed goes past the sync speed of the camera - won't it?
And yes, adjusting the ISO manually is the best bet, instead of letting it stay on auto.