I know it's sensitive to access to user's eye's data, but

Hello @milanowth,

Unity's PolySpatial has HoverEffect GameObject supported, I think it basically means even though I don't know the exact entity that user is looking at. But the developer can provide a event callback to RealityKit that "please change me the entity's mesh to other color". Just like hoverEffect on SwiftUI component.

I believe the behavior you've described here is the visual highlight effect of HoverEffectComponent. This visual effect is not configurable in any way.

So I wonder if there was a closure to let RealityKit the system to fire up my callback?

There is not. I recommend that you file an enhancement request for API that facilitates the functionality you are trying to create using Feedback Assistant.