Accordion groups also allow for an accordion-heading directive instead of providing it as an attribute. You can use that and then wrap your header in another tag with an ng-click.
<accordion-group ng-repeat="group in groups" heading="{{group.title}}" is-open="group.open">
<accordion-heading>
<span ng-click="opened(group, $index)">{{group.content}}</span>
</accordion-heading>
</accordion-group>