Thanks very much to everyone for the sage advise. The event organizers have been in touch w relevant authorities and feel they have this issue fairly under wraps. The homebrew will be gratis for the bands and crews, neither of which will have paid for admission. Tasters for patrons would have been free also, but since they will have paid for entry, I can see how even a free sip might be construed as 'included' in the ticket price. So, will hopefully steer clear of that complication by making it available only to performers and crews, everyone else BYO.
Yes, 10 hl is right - two 40 litre extract batches per day for ten days, boiled a bit over gravity then topped up to 50 l with sterile water as we go into fermenters. It is a much shorter time frame than typical, so will only be making session-strength recipes, fermenting toward the warm end of the range with a high-floc yeast, and forced carbo.