ILRepack VS Fody

ILRepack might be what you're looking for: https://github.com/gluck/il-repack

Combine applications by merging IL, not by storing it in resources. (il-merge, il-repack and unknown to me 'libz', or write your own merger using Mono.Cecil or dnLib)

Depending on which version of .NET you're building with, there are various tools to allow you to statically link all of your assemblies into one executable. Examples are ILMerge for .NET Framework and ILRepack for .NET Core.

While Fody works at binary (MSIL) level, Metalama Compiler is a fork of Roslyn and allows for AST transformations, at the C# level.

As an alternative to PostSharp you may take a look at Fody.

Working with IL weaving as part of the building process is quite complex though, and this is where Fody enters into the picture. Fody is an extensible tool that allows to simplify the weaving process by taking care of all the heavy lifting.

Fodyplugins can modify exists code during compilation.

I've seen coworkers using Fody for such MSIL weaving stuff, but I have a bad feeling about this. It's a little bit like jailbreaking your phone. Cool stuff ahead, but voids warranty.

Implement your own (in C++/outside dotnet/CLR) IHostAssemblyManager stuff, this is the "replace/bypass how assemblies are loaded" trick. There used to be a prebuilt thing on the old old codepages, but I can't find it. I wonder if newer code/assemblie weavers such as Fody can do the similar thing?

Great place to start is https://github.com/Fody/Fody and existing 'addins' - fody does it at compile time - generally quite useful for your own code if you want to dip into aspect oriented programming.