I recently visited the Oregon Country Fair, which among many other things has entertainment acts which pass the hat to earn their living. (OCF only costs about $13 to attend, not enough to pay much if anything to acts.) This is a pretty common setup.
And perhaps this has been done where I haven't seen it, but I was wondering about a solution to what one busker called the "magic disappearing audience trick." Most people don't put into the hat. So along the lines of my microrefunds concept, where I suggest a solution may be to push people into making one decision, instead of many, over whether they will pay for things that don't have compulsory payment, I propose a system for busker fairs.
The plan would be for the Fair to raise their price and provide each fairgoer with "busker chips" to put in the hats of buskers. Once paid for, the chips would, at least officially, be good only for that. The Fair would also probably keep a small fraction of the money, ie. pay each busker 90 cents for each $1 busker chip turned in. People could of course also toss regular money into the hats.
These chips, aside from providing more revenue for the entertainment, would allow the fair to know what acts were the most popular, and thus who to bring back and who to leave out.
There are some other issues to discuss below. Such as the probable black market in the chips, and what price to charge for them...
Left alone, a black market would develop on any chips you force fairgoers to buy. Ie. if you get $10 in chips with your fair admission, buskers who would get $9 for them would gadly give you $6 or $7 for them, something in that range. And likewise, people with chips left over at the end of the day (because they were conservative with them) might want to sell them on the black market or just give them to the last busker they see.
Of course, the fair itself could buy back unusued chips. If so, you would want it to do so at a lower rate, like 30 cents on the dollar, tuned to encourage people to give them to buskers they honestly liked but not so low
that they throw them away at anybody or encourage the black market.
A black market largely would be generated by compulsory purchase of chips with the entry fee. Many street fairs don't even have an entry fee at all, so this is not an option there.
With voluntarily purchased chips, you might want to also provide a symbol which declares "I support the entertainers." This symbol could be a pin or wristband or hat which can be worn by the fairgoer to display they are a "patron of the artists." The patrons get to feel superior, and the buskers can choose to tease viewers who don't wear the patron symbol. Of course the symbol is also a souvenir of the fair. The fair could also provide other goodies to a patron, such as discounts and food booths and other booths, a program, a raffle prize or whatever else the fair has to offer.
While there could be levels of patron, I don't know if that's necessary. Of course the costs of the other patronage rewards can come out of the discount the buskers get. One could go so far as to make this system the entire means of fair financial support. Ie. make purchase of a patronage optional but so stronly encouraged that the majority of visitors do it. In that case the percentage given to entertainers might be smaller but you can't make it too small or the market gets out of whack. If you give people dollar chips it's OK if the busker really gets 90 cents, but not 50 cents.
If, as noted in the comments, there is no black market and a significant number of chips go unredeemed, this could result in a number of things. One, it could just be profit for the fair (and thus further incentive to do this.) (As suggested, this might mean there need be no discount on the payout.) Two, the surplus could be distributed to the buskers according to how many chips they collected. Thus, for example, if only half the chips are spent and half are kept, a busker might get $1.80 for each $1 chip.
There could also be two kinds of chips. One would be an "attendance chip" which you would be expected to give if you spent significant time watching a show. The other would be a donation chip. The former would mostly exist to find out which shows were drawing crowds -- though of course you can figure that out from donation chips and just walking around.
To prevent the black market the fair could deliberately send out trusted volunteers dressed as normal attendees to try to sell chips to buskers for dollars. A busker caught buying chips could face serious penalties (including losing all the money from their other chips!)