I have the official Pi display, and wanted to do the same thing for my portable build. After a lot of digging around I could not find a way to accomplish this.
What I ended up doing was picking up a latching on/off button from Adafruit and a longer length of the PCB wire the display uses. I soldered the button in the middle of one of the wires, and used that for the wire attaching the 5V pins. Then if I wanted to power the display I only had to push the button down, and outputting to the display only took another button press to release. Note if you do it this way, the Pi will not change its video output mode while powered on so it will really only work if you press the button on or off before powering on the Pi.
The default for the Pi is to favor HDMI audio, so as long as HDMI is connected and powered on it will output to that and bypass the speakers automatically.
I am only providing these for reference, you can use any you like.