That's a very cool way to expose the serial port!
Question now is whether the uboot on the device can be made to tftpboot without console access (eg; the asus tftp recovery method), to allow openwrt to boot without soldering.
Also, there's the broader issue of accessing the powerline functionality. There's a previous thread here; Support for Asus PL-AC56 (QCA956X SOC, QCA9558 + PLC-chip) however the poster didn't get anywhere.