Does a similar system exist for .b4xlibs? (so that your 'Custom Classes' come in the IDE menu).
Maybe something one has to add to the manifest.txt file?
I know that, but what I don't know is how to add this in a b4xlib library (the .b4x_excluded files one can add to a .jar lib that produces this in the IDE):
It is currently not supported. You can create a jar file with the template and a xml file with the required declaration. Add the new "library" to the list of dependencies in the manifest.txt file.