Yeah, exactly - they register as a site, then confirm events and have the chair of the society sign off on the attendance. It's a club membership essentially, the group would likely have insurance to cover them the same way that reenactment societies do.
I've notice UKARA are accepting reenactment societies recently as well, I presume they're in the same situation having issues proving their defence.
It's a solution for sure, my concern is how easy it is to falsify - which in turn reveals another loophole and potential lack of responsibility under the VCRA. Of course, it would be the chair of the society who gets the blame - since they've become the auditor in the paper-trail.