Although there seems to be no hotkey to instantly turn on the passthrough camera while playing a game at the moment, the current method I’ve been using while in-game is to:
Click on the Oculus button on the right controller which pauses the game and takes you Home, then to click on “RoomScale” (located above Navigate) to view the passthrough camera. Once I click “Confirm Guardian” it takes me straight back to the game. Hopefully this helps those who need it, I use this method pretty often. Better than nothing at least. Enjoy!