In Rhato rebirth issue #4, Jason mentions 'Dr Quinzel' and having therapy sessions in Arkham, though in the suicide squad arc he seems wary of Harley.
It's an easily ignored reference, and as you mentioned she would have been unable to practise by the time Jason ends up in Arkham.
Though this could also be interpreted as Jason only speaking of Harley in a formal way as Doctor Quinzel since the context of the scene is that he is taking professional advice she offered him once, in a moment where he needs to apply it.
And if that's the case then the reference from rhato could still work if it's interpreted as Jasons interaction with Harley being less of a patient and doctor scenario and more so with the two of them as patients in Arkham, attending group therapy sessions where Harley, no doubt, did most of the talking.