It's much easier to use custom material to bridge metal shader onto Reality model than using LowLevelTexture does the same thing.
Why VisionOS doesn't support this material:
Why VisionOS doesn't support CustomMaterial ?
You are correct that visionOS does not support custom shaders in RealityKit. I strongly encourage you to file an enhancement request for this feature via Feedback Assistant.
Compositor Services does support custom shaders, but is not compatible with RealityKit. Check out Drawing fully immersive content using Metal for more information.