Just thinking of another big festival I go to: over the years the official festival has eventually acknowledged the contribution the festival fringe there makes to the festival by including at least a brief list of fringe events in the official programme. The contribution, apart from the music, is monetary, as official collecting cans are taken to these fringe venues and substantial sums of money forthcoming: of course, pub landlords where sessions take place need to be asked in advance of the festival if they are willing for this to happen. One session in particular usually puts the can round once per hour in a 3-hour session: no compulsion to contribute but works far better than just leaving a can on the bar and hoping people will put money in.
I take the blame for introducing collecting cans to the festival I used to help to organise: we had to get a street collecting licence for collecting at our outdoor stage, and I checked all the session pubs in advance as above - only one refused, saying they already had other charities they collected for.