Most ISPs I am aware of only block a small handful of commonly abused ports for outbound connections. A much bigger problem is blocking inbound connections. A real ISP (like CenturyLink) will sell you a static IP address that is completely unblocked, for a nominal fee.
I don't see how anyone can expect to operate a Tent server without such cooperation, so the protocol used for server-server communications is almost irrelevant. It is the client-server protocol where special consideration needs to be taken, and that will naturally be a web interface in most cases.
The idea that HTTP provides some sort of filter advantage for server-to-server communication, however, seems to be entirely a red herring.