I believe they have some sort of similar solution in place for free messaging on in-flight wifi. Having a look at how they've done that might give a better idea for what this will look like than speculation.
Still, it's a hard problem. They want to prioritise connectivity on an extremely low bit-rate channel that covers an extremely large geographic region. Messaging apps needs to be a part of that connectivity solution, but you can't have one user making the entire cell unusable by everybody else. I don't know if there is a 'net neutral' way to do that, but would love to hear your ideas.