I use a random 63 character WPA2 password, and my solution was to cable it initially, and then set up the WiFi password using the iPhone remote app.
And if all else fails, entering 63 characters is not really that hard either (unless the Apple TV has one of these weird on-screen letter-choosing wheels you sometimes encounter in videogame consoles and the like).
Oh god, yes it is. Especially when you realize halfway through that the iPad cannot type "`".
(You can copy and paste it, though.)
Or Roku or a nest... Tons of devices use wifi but lack cameras or copy/paste.