a while ago I found out that you could do this by running a tor hidden service on the device. If I needed it, I think I'd rather use that, or a reverse tunnel than going through a third party.
I've recently developed a utility[0] to help simplify setting up hidden services w/client auth for this purpose, you might find interesting.
via reverse proxy: `ssh -NT -R 1234:localhost:22 youruser@yourmachine.com` Now if you connect to youruser@yourmachine.com you can then ssh to your inaccessible machine with `ssh inaccessibleuser@inaccessiblemachine:1234 `