I successfully connected a cheap Arduino Mini Pro 3.3V via serial connection to a cheap NEXX 3020F mini router running openWrt:
Even connected GPIO2 to DTR so i can reset the Arduino from the Nexx.
And i think, all it needs to install a new hex file is to send the binary immediately after such a reset.
Maybe there is already a lightweight linux solution like http://russemotto.com/xloader/
Obvious idea, i want to put a lua webpage onto the Nexx to upload a lua script for the Nexx and add a hex file as attachment to be installed on the Arduino which will take care of all the io. All wireless