I managed to get my first extension board working for Beagleboard X15.
I wanted to be able to use some LCD panel with video out extension port (P18), this can be easily done with TI SN75LVDS83B transmitter, which I integrated into this design.
I also added some GPIO on a standard pinout to add some hackability to the board.
Schematics, layout (eagle) and some doc are available here https://github.com/julbouln/x15_ext1
I should add that the Hirose connector is a really good choice for extension. It seems robust at usage, and soldering it is actually very easy.