The pattern given here suggests that these files are not packed into a jar from
the compilation result. Thus, the annotation '(cannot exclude class files)' made
me wonder why. In fact, this was exactly what I wanted to do: exclude all
classes that appear in 'sandbox' packages from the distribution.

Looking into the build-impl.xml, I found that this property isn't actually used
for packaging but for compilation:

    <target name="-do-compile"
depends="init,deps-jar,-pre-pre-compile,-pre-compile" if="have.sources">
        <j2seproject3:javac/>
        <copy todir="${build.classes.dir}">
            <fileset dir="${src.dir}" excludes="${build.classes.excludes}"/>
            <fileset dir="${src.res.dir}" excludes="${build.classes.excludes}"/>
        </copy>
    </target>

I suggest to either (or both) move this setting to the 'Compiling' department of
the project properties ('Exclude From Compilation') or to evaluate such a
property in the jar target as well. The latter is what I need.