VS2022 .vcxproj.user UserMacros properties not applying to .vcxproj fields

Question

Hello! I am relatively new to understanding the .vcxproj schema, but I cannot find solutions for what we want to do.

We are programmatically generating our .vcxproj file with a custom NMake build commandline. I want individual users to be able to append commandline args from a .vsix extension.

To facilitate this, I had the idea to add a custom macro to the generated commandline in the .vcxproj file (so that the .vcxproj file itself remains static). This works if I define the custom macro inside the .vcxproj file itself:

<PropertyGroup Label="UserMacros">  
    <DynamicVSIXBuildArguments>-MyAddedBuildArg=Value</DynamicVSIXBuildArguments>  
</PropertyGroup>  
...  
<PropertyGroup>  
    <NMakeBuildCommandLine>../Path/To/My/Build.bat MyProjName $(DynamicVSIXBuildArguments)<NMakeBuildCommandLine>  
    ...  

However, if I move the <PropertyGroup Label="UserMacros"> block into the .vcxproj.user file, the macro doesn't resolve in the build commandline (it's resolved to nothing). I even had the vsix extension writing the property to the user file using this code:

IVsBuildPropertyStorage PropertyStorage = ...  
...  
PropertyStorage.SetPropertyValue("DynamicVSIXBuildArguments", null, (uint)_PersistStorageType.PST_USER_FILE, "-MyAddedBuildArg=ValueFromVSIX");  
								  

And by default that writes to a "UserMacros" property block without me having to specify it.

I know I was kind of blindly hoping this would work, but would like to understand why it doesn't, and how I might achieve my original goal.

Thanks!