But in the rock/pop/indie band circuit, the idea that there could be a whole network of venues where ANYBODY AT ALL would turn up and pay to hear somebody they have never heard before is pie-in-the-sky territory.

Pie in trhe sky? Hardly. We've got such a place in Harlow ( here, and I don't think it's untypical of what exists elsewhere. And of course there are also pubs which regularly put on unknown groups.