Set up a redstone clock with a command block and set it to repeatedly teleport the player locally down a number of degrees. (I used 1, my command was /tp [playername] ~ ~ ~ ~ ~1)
Look down, then open the chat menu.
The view will keep teleporting past the allowed amount.
(Note: I did NOT use the upside down shader.
Also, you MUST open the chat menu.)
Oops, duplicate of MC-45115. How to remove?