IMHO, It's just plain stupid thing to do. Not because of your idea technically speaking, but because of making everyone else concessions over something which is purely software based (installing some binary packages system wide). Yes, shared server is a pain in the ass for Phalcon, but what can you do, those clients are usually fine with a raw php or whatever other framework/cms out there.
That's why Phalcon have such good reception in an enterprise environments, where you have the freedom of choice to setup everything etc.
At the end of the day, if some hosting company would want to support Phalcon, it's not a hard task, just like they maintain their other .so binary components, i.e. CURL extension or PDO etc. They are not there by default...
Task of converting zephyr code 1:1 to native PHP could be done, but such task is cumbersome to me.
And if you still want to try such thing, this is a good reference for start:
https://api.phalcon.io
Take a note on "Source" button located on every class/namespace. It's really useful. I copy/pasted some parts of some classes out there in order to override them while keeping the same interfaces.