Calm down lunt it doesnt have to be a elytra like maybe a secret air class were you have to give up all your worldly possesoins in exchage for magic and flight or some magic artifact tell me plzzzzzzz
So flying mounts would be a no-go. On another subject, can you bring mounts into a dungeon? If your mount is like your pet (or if it is your pet) then can you bring it in and fly it around or something? That would break the entire premise though...
How about levitation potions or something along those lines, sure you wouldn't be able to control that, but you could essentially float to the ceiling (in specific circumstances) and avoid the entire parkour aspect of the dungeon.
Any loopholes would have been found by the server and rendered useless, so you can forget any loopholes around the flying thing. Most likely even Float enchantment isn't going to work