Well, whenever I didn't need to heal/buff/support with Rosa and Garnet before they got stronger attacks (Holy/summons), I just pressed Attack. Garnet did low damage (don't remember how much Rosa's arrows did), but that was better than doing nothing.

Anyway, I would be fine with renaming healers to something that reflects their actual role of Attacker with healing and support actions. Maybe that would silent the people who say "I queued as 'healer' and thus I only heal".