So this is something I had in mind. Has anyone ever tried to boot a custom OS from a USB drive with Roku? (no matter which version, my house has pretty much all of them).
Is it possible to do so? If not what's the reason?
It's not possible. Running custom firmware is not something Roku Inc approves of, so they have locked down the devices.
Apparently Roku was penetrated last year but they fixed it within couple of days, see Exploiting Roku