I would like to use openWRT-smarthome on other hardware platforms (e.g., ramips). How was short_delay determined and is it likely to change on another platform? Can you think of a more portable way to set the timing?

Maybe the code from https://rc-switch.googlecode.com/svn/tags/v2.3/RCSwitch.cpp which uses pinMode, digitalWrite and delayMicroseconds can be rewritten to use your /sys/class/gpio/ based functions. This would allow for arbitrary commands to be sent, and more types of switching plugs to be used (this part is already figured out by the RCSwitch code)

The first argument to the digitalWrite and pinMode functions are supposed to be the GPIO number, but nevertheless gpio29 is hardcoded.