It would be great if you could expand your explanation of the metaprogramming into a comment in the code. When I read it, I didn't immediately recognize the technique and its tradeoffs, so it seemed like an odd decision rather than an informed one.
Having two versions of phlex would be a strong negative to me. I don't like to see dev teams spread themselves out, it's one more choice I'd have to make when adopting, if switching between them I have new risks that they've slipped out of sync in some way, and it's altogether another odd decision that has to be explained.
What I was trying to say about the code is that I share the previous commenter's experience of "welp I guess if we want that bug fixed we have to click 'fork' on github... while we write up our plan to migrate away". Having had that experience several times, Phlex looks like an acceptable risk.