Yeah, I haven't looked into that much because everything is on my local network. ngrok looks like it would work. One of my users is looking into remote.it, which seems like a similar service.
I work for remote.it. If you or your users need any help setting up remote access to the TinyPilot let us know. This looks like a great application. We can make ssh/VNC/http on the Pi available remotely over internet as a free service no problem.