Access to previous field should probably be synchronized:

    @Override
    public void addHooks(HookRegistry hr) {
        hookRegistry = hr;
        hr.addClassLoadingHook(this);
        hr.addBundleFileFactoryHook(this);
        hr.addAdaptorHook(this);
        if (configurators == null) {
            configurators = Lookup.getDefault().lookupResult(HookConfigurator.class);
            configurators.addLookupListener(this);
        }
        resultChanged(null);
    }

    @Override
    public void resultChanged(LookupEvent ev) {
        Collection<? extends HookConfigurator> now = configurators.allInstances();
        Set<HookConfigurator> added = new HashSet<HookConfigurator>(now);
        added.removeAll(previous);
        for (HookConfigurator hc : added) {
            hc.addHooks(hookRegistry);
        }
        previous = now;
    }