I am working on 41 instances right now and handling the “Instance Preview” custom parameter is a bit cumbersome. I want the same string for all instances (for comparison, naturally) and updating it for all instances individually feels like an unnecessary effort (I ended up editing the .glyphs file in a text editor).
Maybe “Instance Preview” could be supported as a custom parameter under Font Info -> Font? It could still be overridden if specified for a specific instance.