The first time I wanted this was when someone gave me a usbarmory [0]. It easily fit under my thinkpad's palmrest, but was of limited value for secure computing as the only way I could interact with it was via things like ssh / X over ssh, and I was still displaying/handling all of the unencrypted information on my relatively insecure network-connected host that runs web browsers and accesses all sorts of untrusted input like porn sites and message forums and what have you.
I was able to leave my private keys on the usbarmory which is a marginal improvement, at least nobody could easily sign things as me for instance. But for reading/replying to encrypted emails, I couldn't make the usbarmory connect to my keyboard and display directly, it doesn't have pins for such things nor do I have a micro KVM switch as you mentioned.
> $229 (excluding the pi)
> Laughs in $100 ThinkPad.
Seriously though this looks great. And from what I gather it works with lots of newer phones, and can even be used as a second laptop display.
That way the Pi can serve as a secure computing environment sharing the display and keyboard of my insecure laptop.
On top of that a bunch of neat integration can be added like the insecure laptop submitting ciphertext to the attached Pi with a seamless switch of keyboard and display over to the Pi where I can view the plaintext or do other operations on it, and have new ciphertext sent back to the insecure laptop with a seamless switch back of keyboard and display. I'm imagining integration with tools like mail reading software etc. for doing the switching.
Unfortunately USB-C alone isn't going to enable this level of integration, but I'm optimistic projects like the mntmn reform can facilitate this area of innovation in the future.