I've finally been able to expose custom properties to a Lumira visualization extension by following the tutorial here:
Visualization Extension Plugin for SAP Web IDE Guide
My question is this - Does this only apply to the 'VISUALIZATION PROPERTIES' area in the 'Compose' room? Is there any way to also expose it in the toolbar or elsewhere in the 'Visualize' room? Is there a special property zone or something else that I'm missing?