See, e.g., this, from their FAQ. At the end of the day, it's probably "stuff the FCC doesn't approve of". But my interpretation is that syncing Bitcoin nodes would be OK, since you're not receiving compensation. But routing fees would be verboten, even if encryption were allowed.

What might be interesting would be broadcasting blocks and/or transactions a la Blockstream's satellite service. Or, if that's too bandwidth-intensive, maybe setting up a service where your node could accept transactions submitted via packet.