Classpath containers doesn't work like you suggest unfortunately - they can't "extend" Ctrl+T without adding stuff to the actual classpath.
But:
Henk, go to Eclipse preferences, type "default classpath entries", locate JBoss 7.1 runtime and add modules/**/*.jar and you have all the jars you want.
Its not perfect (since its currently not possible to default to this) but we've provided this "loophole" until we get a way to avoid duplicate jars.