This idea has been abandoned. It does not appear to be possible to accomplish this with source generators. Source generators are not able to modify/delete existing code, thus I do not believe it to be possible to inject throwing an argument null exception inside of a method that has a parameter decorated with a specific attribute.
I am keeping this repo around though for future reference by myself and the community.
A C# source generator to generate throwing an ArgumentNullException for any method parameter decorated with a specific attribute.