How do I output messages to the Visual Studio output window, from MSBuild?

In your project properties → Build Events, you can add something like this in the Post-build event command line:

echo This is my message, no quotes required!

And then you'll be able to see it in the Output after a successful build (if configured to run on a successful build, which is my case).

This may help:

Under Tools → Options → Projects and Solutions → Build and Run, there’s the MSBuild project build output verbosity combo box. This controls how much info you want to see in the Output window.