Wow, thanks, guys! I thought this idea would be panned, but it's actually already implemented, cool! So, at least I don't feel silly to try it.
Just in case, my use case is to establish communication between two parts of a DIY keyboard. I wanted it to have a nice looks, so, I'm going to use ready-made 4-wire cables, either TRRS or RJ9. Too bad I didn't find nice curly cables with plugs and sockets for 5 or 6 pins.
Although, chances are I'll end up with i2c because it's easier

. But for the sake of experiment I'll check SPI as well.