Note also that AdditionalIncludeDirectories is per file, as it can be changed for individual source files as well as on the project level, and if it contains macros it can evaluate differently for each source file too!
I'm not familiar with driving the MSBuild objects via the API, but that's used by the IDE. Whether that way or by simply running MSBuild.exe, you need to get it to figure out all the properties, conditions, etc. and then tell you the result. If everything is well behaved, you could create a target that also uses the ClCompile item array and emits the %(AdditionalIncludeDirectories)
metadata somehow such as writing it to a file or passing it to your other tool somehow. That's what's used to generate the /I
parameters to CL, and you can get the same values.
If things are not well behaved in that necessary values are changed during the detailed build process, you would need to get the same prelims done just like the ClCompile target normally does, too. Or just override ClCompile with your own (last definition of a target is used) so it certainly is in the same context.
Either way, there are places where build script files can be automatically included into all projects, so you can add your stuff there or use a command argument (I think) to MSBuild to add another Include.
—John