Like this?

https://alexanderell.is/posts/wikipedia-over-whatsapp/

https://news.ycombinator.com/item?id=31463249

I'm sure I've seen what you're describing done before to make use of free WhatsApp in mobile plans in some countries, just can't find it, although I remember that one of the limitations found was rate limiting by WhatsApp, which is to be expected.

Those countries with only WhatsApp available for free over internet should do something about net neutrality. When the telcos can sell app bundles, they will not focus on acting as an infrastructure provider.

or DNS port ? i remember some captive portal would block all traffic except dns request until you login.