I've been saying the same thing for years. 9p makes for a great control protocol that presents a simple way to present data, a tree of files.
Charles Forsyth wrote an Atmega compiler for plan 9. That could be used as the basis for a plan 9 Arduino thing. We likely need a mulibc.h or whatever and a loader to send the binary to the Arduino micro controller. Someone patched 9front usbserial to recognize Arduino USB boards as usbeia devices. (I think Miller's pi and labs does too?). Out of the box microcontroller support with 9p libs would be a really nice thing to have.
Echoline wrote ninepea, an Arduino 9p library. This currently works and can be mounted on a plan 9 machine and you can talk to your Arduino though files. Though this uses the official Arduino library and compiler which does not work on plan 9 (on 9front you can use vmx to run a Linux vm)
One compiler that we really need is an arm thumb 2 compiler for cortex-m/r microcontrollers. Inferno has a very old and very incomplete arm thumb 1 compiler, tc. I don't know if it's worth salvaging.
Personally, I'm (sloooowly)working on using Plan 9 for personal industrial automation projects. I have some crude modbus stuff working.
At work I've used it on a RasPi to tie a PLC to an HDMI heads up display that uses ASCII commands. Dumb easy to write a C program for that on Plan 9.