There is no problem to reset to factory defaults with LAN cable.
But is there any , just any (not JTAG/serial though) option to reset OpenWRT to factory default without LAN cable?
I got stuck in the situation where I need it, but I have zero chances to get twisted pair in a reasonable amount of time. Remote, VERY, VERY VERY remote location (kind of end of the world) and router with WiFi disabled in config.
This router need to be configured to share (slow) internet from USB modem via WiFi.
You can naturally reset the device also using a wireless connection, but then you lose all config items that are not in the flashed firmware image. In practice you usually lose also wireles config and connectivity. Also possible wan firewall settings etc.
If you still have access to the device, you could also tailor the needs settings at your buildhost and embedd the needs settings as custom files into the compiled firmware image.
But why do you actually need factory reset?
Most default settings can be found from /rom/etc/config directory. But network and system config files are not there.