Would this allow a satellite internet add-on if you find yourself going somewhere seriously remote? If so, does such a satellite internet peripheral already exist?

I would guess it's at least possible, but not sure if it would be practical.

The first (duplex, it seems in satellite it still makes sense to offer simplex data) modem I found was this: http://www.globalstar.com/shop/index.php?main_page=product_i....

Of course there is no price, and I find it mildly scary that the US sales contact is listed as "USA/Government", not just USA.

This thing has a 20-pin header which they claim includes data connections, but there is no pin-out. There is a PDF (http://www.globalstar.com/en/docs/gsp1720/GSP-1720%20Sell%20...) but it's more of a brochure and doesn't provide a lot of detail.

The bandwidth for data over satellite modems is low enough (10s of kilobits/second) so I guess it seems reasonable to do over GPIO, though.

It would allow for this. We're talking with two different satellite providers about what this would need to do. The biggest problem seems to be that the satellite connections are pretty good standalone systems, but they don't do a good job of making a WiFi station, what would be helpful is a self-powered router to connect to - and this is where the BRCK sits.

We recently ran a test of the BRCK up in the far northern reaches of Kenya, where the closest mobile phone tower was 160km away. The only option for internet was satellite, so we hooked up a BGAN terminal and ran that ethernet line into the BRCK to broadcast the signal as WiFi. (more here: http://www.brck.com/2013/11/brck-eclipse-day-7-home-again/)