The process was simple: find, in the NetBeans sources, the package containing the code for the Window Component wizard, copy it into your own new fresh module, tweak it, and voila:
It's not really a non-GUI window, of course, but a non-Matisse window. But let's agree to not quibble and split hairs at 3AM, OK. When you use the template above, you get exactly what you'd normally get, except no ".form" file, nor the code referencing it, i.e., the "initComponents()" is removed from the generated ".java" file.
A future version of NetBeans IDE should have the above functionality integrated into the Window Component wizard, i.e., when you create a new window, you should be able to specify whether Matisse support should be provided.