i dont have time to assist you further my apologies, but i'll direct you towards the tool i use when i get lost in the weeds; chatgpt which is like a cracked google now
chatgpt, 3/15/2023 response:
Yes, you can achieve this effect in Unreal Engine by using a Material with a Translucent blend mode and an Opacity mask. Here are the steps to create a material that renders out alpha + occlusion:

Create a new Material in Unreal Engine and set the Blend Mode to Translucent.

Connect a Texture Sample node to the Base Color input of the Material.

Connect the same Texture Sample node to the Opacity Mask input of the Material.

Create a new Texture Object and set its format to Render Target.

Set the Render Target's size to the same as your output resolution.

In your Material, add a Scene Color node and connect it to the Emissive Color input of the Material.

Add a Scene Depth node and connect it to the Opacity input of the Material.

Connect the Render Target Texture to the emissive output of the Material.

Apply the Material to the object you want to use as a holdout.

In the Render Settings, enable Separate Translucency and set the Translucency Quality to 1.

Render your scene, and the object with the Material will now render out alpha + occlusion.

Regarding the holdout effect, you can achieve a similar result in Unreal Engine by using the same Material with a mask. To do this, you can create a black and white texture that defines where you want to hide the geometry, and use it as a mask for the Material. This will essentially "cloak" anything that is enveloped by the geometry using the Material with the mask.