Re: Nice!
"you take a phone, zero it and install your phone OS of choice"
This is possible, it's just usually your choice of OS is AOSP-based or bust. Getting a minimal Linux install running should be relatively straightforward, but it won't be much use as an actual phone.
The problem isn't being able to run the code of your choice on a phone (several manufacturers allow you to unlock a phone direct from their website), it's that there's very few mobile operating systems, and even fewer open source ones.