I think libgccjit uses this technique or can use it: it reminds me of things I read about the elisp JIT in newer versions of emacs

There's Deegen, which is being used to build a LuaJIT successor. https://github.com/luajit-remake/luajit-remake

From what I gather, Deegen is independent of the VM in that repo, but lives there afaik.

I mailed the authors a few months ago and indeed, it lives there but is independent-ish. They don't consider it ready to use for anything else other than experiments, though.

It is utterly baffling that people are downvoting this comment.

The repo I link to here is by Haoran Xu, one of the authors of the paper.

Y'all need to chill out.

Blog post about the approach here: https://sillycross.github.io/2023/05/12/2023-05-12/

Cpython in beta

Sure but from looking at the patches, it seems like theirs is tied into the CPython build and infrastructure. I am talking about a dedicated library which can be dropped into a variety of superprojects to help them implement a JIT.