“If the wifi is letting WhatsApp messages through, what if we used WhatsApp as a vehicle for the information we really care about?”
Delta’s free in-flight wifi lets messaging apps through and blocks the rest. So send a WhatsApp message to a bot, and it replies with the Wikipedia article in 20 chunks. Any filter that allows text out will carry anything you can encode as text. Paywalled wifi has always been a policy, not a wall.