And then I remembered the Koala Layout created by Jean-Marc Borer and others (from SkyGuide in Geneva, where they make air traffic control software on the NetBeans Platform):
Go to this site: https://koalalayout.dev.java.net/
And there you will find a NetBeans plugin for providing Matisse GUI Builder support for Koala, also created by Jean-Marc:
At least you'll be able to learn from those sources (hope that project will be migrated from java.net to the new infrastructure). Would be really cool to investigate MigLayout support, for example, via the sources in Koala Layout. I was trying to do that some time ago but never got very far with it.
Anyway, anyone intending to integrate their own layout manager into the Matisse GUI Builder should be aware that the above starting point exists.