OpenWrt

OpenWrt as the site says is a Linux distribution oriented towards wireless routers. One way of designing an OS for an embedded unit without a hardware reference is getting processor specs(by opening the unit), compiling code and creating a binary image compatible with that processor which is downloaded(via tftp) to the router. If the router has a usb/serial interface debug output could be spewed through them comparatively easily. Once that is accomplished additional hardware can be detected and made visible to the developer through the debug interface who then has to hunt down the chip manuals for the hardware and write code to deal with it. Pretty simple isn't it :)