Here's what you can try with the 7D: In the camera's recording menu, set "flash mode" to "fill-flash", "flash control" to "manual", and set the power ratio to "1/16". This does two things: Any pre-flash is turned off, and the intensity of the built-in flash is set to the lowest setting. Also set the ISO value to a low number, e.g. 100 or 200. Both together will result in little exposure from the built-in flash. Again, this small flash burst may be enough to trigger a studio flash system. Of course, with a digital camera you can see the effect of the built-in flash and of the studio flash immediately after the shot. If necessary, you can further reduce the intensity of the built-in flash with filters or a piece of paper.