This evening I toyed around with my now deprecated wireless router ZyXEL NGB6515. After opening the case there is a distinct 4-pin connection point for the serial console. The pins being 3.3V, GND, TX, RX (3.3V is the square pin).
Connecting a USB-to-Serial dongle set to 57600 bauds yielded a log from the main MediaTek MT7620a processor. Right after I connected the 12V power there is a short period when the bootloader (U-Boot) waits for an integer input to choose which mode of operation the device should run in (e.g. flashing, OS boot etc.).
By default the system boots a 3.2.9 Linux kernel. Though I couldn’t come up with a way of controlling the command prompt. There is a “cmd>” prompt, but it ignored any input and just reprinted the prompt on every newline.
Sadly this device is not supported by OpenWRT, even though the MT7620a processor is a pretty standard chip among home routers.