2.92 alpha, how to use custom object properties?

I’ve tried adding a custom property named LightScaleY both in Light Object Data Properties/Custom Properties and Object Properties/Custom Properties, and I’ve tried selecting Geometry, Object, and Instancer for both types - to no avail. How is this thing supposed to be used?

I’m basically trying to use area lights of different lengths to appear as light strips with evenly spaced leds.