But there's a problem here -- the way this is used in practice pretty much locks addicts out of the possibility of treatment altogether (I mean, unless they're functional enough to cover up the addiction). And treating *only* the addiction, when someone is in part self-medicating for underlying mental health issues, can also fail pretty badly.

I can definitely understand requiring that someone be *concurrently* in an addiction treatment programme in order to access therapy (as you recommend). But what usually happens is that people are required to be clean for a long period of time before they're allowed to access therapy at all. This model, to put it bluntly, sucks, and is one of the many reasons there are so many mentally ill addicts hanging around my church.