So, so awesome. I hope more hardware companies start doing this.
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.
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/)