Alright well, that's about 72 hours of building, retrying things, and pulling my hair out. Here's what I've managed to sort out;
Using the Unreal Engine 5.3.2 from the Epic Games launcher:
Download the MetaXR plugin from here: https://developer.oculus.com/downloads/package/unreal-engine-5-integration/
Download the MetaXRPlatform plugin from here: https://developer.oculus.com/downloads/package/unreal-5-platform-sdk-plugin
Both of these should sit in your Unreal Plugins folder under a folder called "Marketplace". These should work with the latest version of Unreal, but you don't get all of the sweet bells and whistles from the Unreal Meta Fork
I haven't tested the Entitlement check on the MetaXRPlugin so I can't speak to that.
Using the Unreal Engine 5.3.2 Meta Fork:
Downloaded from here https://github.com/Oculus-VR/UnrealEngine
and installed and compiled painstakingly over (for my computer) 4-6 hours.
Once I compiled the version I got from the link above, it was missing the MetaXRPlatform. I tried to install the MetaXRPlatform plugin like I did by putting it in the plugins/Marketplace folder but that didn't work for me.
What did work for me was getting the MetaXRPlatform version from here: https://github.com/oculus-samples/Unreal-SharedSpaces
After trying to put that version in the marketplace folder it also didn't work. So I left the files in the marketplace folder, and then opened visual studio. I added the Folder structure via "add filters" in the context menu and dragged the files from the folder into the right places. Then I built the solution again. Thankfully, VS is smart enough to only try to build things that need to be built.
I tried this same process with MetaXRPlatform version 65.0, 64.0, 63.0, and 62.0 to no avail. It was only the one that I downloaded from the link to 'shared-spaces' that worked for me. It built successfully and when I started up my Unreal, the MetaXRPlatform was finally functioning in the version of Unreal that Meta modified and that I had to build on my machine. I'm currently testing the Entitlement check. I'm not 100% sure I have it working, but it looks like it's doing a check.