If you're talking about acute biological withdrawal, often that will manifest in the police lock-up over the first couple of days after arrest and in the provincial receiving remand prisons, rather than at the federal level. From what I've seen through the contact I've had with those cases locally in Toronto, the health care staff in the prisons are on to that as a risk, and the primary mental health and drug and alcohol screening and general practitioner services there are certainly looking for those things.
In terms of addiction counsellors being available to follow up from there, I think they are thin on the ground, from what I've seen.
Of course, remand prisoners particularly are in very unstable situations. Soon after you've been arrested, you don't know how long you'll be in for. It's an entirely new world—or it may be a familiar one for you, if you're a frequent flyer. For some of those people, engagement at that point is really very important, particularly—and this is a provincial rather than a federal issue, I guess—for the rapidly turning-over remand people, who often have less serious offending but may have major drug and alcohol problems. Getting those people to turn to drug and alcohol treatment at that point is something for which you would get a good bang for your buck.