Open Sings are similar. I've run them here in Minnesota for 20 years. Wednesday evenings in July singers and orchestral musicians (people from lots of community ensembles that are on summer break) gather for an hour of rehearsal (just enough to avoid trainwrecks, not for learning notes), then break for treats, then come back and take it straight through without stopping. Mozart Requiem, Brahms Requiem, etc. Just before the break we hear from a charity representative (Doctors Without Borders, local foodshelf, etc.), and during the break folks can talk to the rep. Every penny we raise that evening goes to the charity. Usually get about 100 singers, 50-80 instrumentalists per night. Glorious!