Got round to adding the text to direct real humans to change the forum style to proceed with registration.
I tried to debug the issue first, and couldn't figure it out. At the rules acceptance page, if you switch to Old Style and then back to vB3D 2 (or LC), you get the reCAPTCHA there. Seems something happens when displaying Old Style that triggers it showing up.
Anyway, new users should see the text directions.