very nice project that you are doing.
It looks from your wiki that SPI is currently used? Interesting that the board can be programmed as well as just interacted with.
That's an awesome thing for enabling remote firmware upgrades to the MCU if this setup is tucked away somewhere.
There is a 2x7 connector in the Dragion. This connector includes: SPI interface, UART/serial interface, 3.3v, 5V.
you can use 3.3v or 5v to power the Arduino and use SPI , UART to communicate with Arduino. The SPI can also be use to remote upgrade the MCUs via avrdude. A pin map of Dragino is here http://www.dragino.com/MS12/architecture.html
But perhaps rather an doughterboard, since this could flash anyway, perhaps an Arduino (well ATMega328 really) could simply be added to the PCB itself, SPI vias etched out on the PCB, and its GPIO exposed externally somehow? I don't know how often you'd really need to replace a daughterboard if this setup could allow you to upgrade firmware via the router itself.
Our plan is to develop a plug in Arduino board (like Duemilanove), then provide Dragino and this Arduino plug board as a development kit. i think this will let developers more easy/friendly to use for their development. The post here is get the ideas what feature of such board should have.
BTW, can your wireless SoC allow for the router to act as a Wifi client and expose a bridged AP at the same time? This is a really great project!
not quite sure if OpenWrt support this, but i know someone already use the same wifi interface to run Ad-hoc and Wifi client together.