The model you describe is very much like the Ark in Ann Arbor Michigan. It is a great venue, but has evolved over at least 30 years. there are still open mike nights, and I know of no other clubs, that have emphasis on drop in performers. I am limited to the role of audience member, so I am OK with other people, paid and planned or spontaneous, being on stage. There used to be a group of "old timey" musicians in East Lansing, MI. that were playing on a weeknight each month, in an alley off of Grand River, (main street through Mich State U.)They were loosely based at Elderly Instruments, don't know anything else to add.