I'm working on it right now and have everything except for the human verify working in 3.8.2.
So far my process was to edit the STANDARD_ERROR template and insert most of the register template into it.
I've been testing it for functionality against newpost.php, i.e. visiting the site as a non-logged-in-user and replying to a thread.
For testing I've inserted the require_once code from this thread:
https://vborg.vbsupport.ru/showthread.php?t=182435
Where I'm at is that everything works except the recaptcha doesn't display. Hopefully I'll figure this out soon, but if anyone has a solution as to what I might be missing I'd appreciate it.