|Image from Wikipedia|
As I wasn't there, I don't know how the artists would have reacted. I do know that on other occasions when the audience has not behaved as expected, the answer has been 'not very well.' This was certainly the case in one of the early performances of one of Stockhausen's more approachable pieces, Stimmung. In the piece, lasting about an hour, a cappella performers sing a single chord. However, it is a genuinely interesting piece because they vary how they sing the notes throughout - using different octaves, sounds and words, tones - I really rather like it. At the performance in question, the audience members started to join in, singing in their own notes in the chord. Now, to me, that's brilliant. But apparently those involved (I can't remember if it was Stockhausen himself or just the performers) were furious and stopped the performance.
As I quite regularly go to Bristol, I'd also say the same goes for those who add things to Banksy paintings. The whole concept of painting on walls is breaking the rules - so you can hardly complain when someone else does the same thing. In some cases where a Banksy has been 'defaced' I think the result is an improvement. In others it's borderline. The image shown here has according to Wikipedia been 'defaced with blue paint'. Actually the 'defacing' is quite effective as it looks as if someone has shot at the people with a paint gun, which itself could be interpreted artistically (in fact, I didn't know it was 'defaced' until I looked it up). Admittedly if all someone does is scrawl a tag over it, it's not a great contribution. But even so, I'm not sure we have any right to complain. If someone does it to a conventional piece of art in a gallery, that is totally unacceptable. But if you really want to be avant garde, then you should go with the flow when it comes to others contributing. Get antsy about it, and it shows that underneath you are still very conventional.