@luz Thank you for a great and useful reply!
As it is a MT7688 hardware feature, you need to configure it at that level. The omega2-ctrl utility has a option to change this - I never used it myself but I guess it should do the trick:
omega2-ctrl gpiomux set ephy gpio
That command will completely disable Ethernet PHY, isn't it? It is the way, but I already use EPHY in my design, so no.
So I personally would not use GPIO43 for a hardware function that could cause real-world problems in case it suddenly receives ethernet activity flicker...
When I was designing a PCB for Omega 2S, I haven't had all the docs, and as a result I've used some of I2S pins, almost all SPI pins, etc. (literally, all the pins that could cause boot and other problems when they are pulled up/down) as a GPIO. Now I have a bunch of the produced PCB's, and yeah, I've gone the hardware way - "cut the route and solder a wire instead". Yep, I know, that this is the shittiest way to do the thing, but I don't want to burn something out in case of some SUDDEN ethernet activity.
Also, thanks for the MT7688AN datasheet, it has a lot of useful info!
And particular thanks for the devmem example, and comments, that's just amazing!