I would like the 'rg_no_new_regs' hook; which gets called when the forum is not accepting new registrations (and the user clicks register).
Here is the patch: http://karpenoktem.nl/~jille/punbb-rg_no_new_regs.diff
(I already posted this to some bug-reporting tool; but that seemed to be only for critical/security bugs. I'm sorry for that)