A problem for a big ticketed rally for sure, but probably less so for an invitaion only, private function held on a commercial property.
I know of a couple of conference locations that run Out-of-bounds type team building things that allow groups to have fires and I've found a few campsites that have fire pits. I assume they take care of their guests legally. This is as far as my idea has got, so I don't know if such places will have much time for a bunch of strapped for cash bikers in January who are maybe going to bring an MZ powered air-raid siren. Given probably a fair bit of time I'm going to do more research, but was thinking of a "fancy a beer in X, bring your tent they have camping, it's £?? per night" type job. Obviously it'd be a different asmosphere to a Dragon/Elefant (no badge, no coaster, no soup, bring your own Whiskey) but the Germans seem to be doing these "micro" rallies too and they might perhaps fit what some people want to do?
Andy