What this boils down to is money.
If we do a single, organised homebrew competition, on a weekend say, we will need a venue and that will cost. That in itself isn't a problem, but it means we need to charge entry to cover costs.
Where we run into trouble is when people get to drink untaxed beer after paying for entry. It could be argued that you are effectively paying for beer when you are paying entry and if there is no other benefit to entry I would be very surprised if a judge would see it as anything else.
To do this, we would need to figure out a way to have this little gathering without charging people in, or without giving people tax free beer to drink.
It's a tough one.