How about limiting the amount of visitors at an exhibition by only selling tickets that allow entry at a certain time and for a certain duration, for example 9am for 2 hours? This would spread out evenly the amount of visitors visiting the venue evenly throughout the day and you could limit the number of visitors evenly throughout the day. If only 200 visitors can fit in the Ally Pally at one time with 2 metre social distancing for example, then only sell 200 tickets for 9am entry (perhaps at a premium) then another 200 tickets for 11am entry and so on. Shows could open earlier and end later to allow more ticket sales.
Obviously on the door tickets would not be possible with this but I don’t see this prohibitive per se. Many book in advance for these shows anyway and security could be hired for any troublemakers. We must try to find a route back for railway exhibitions to once again take place.