I know its an old thread, but googlin around I found that the CBD is an allosteric modulator of mu and delta opioid receptor, meaning that its bind on that receptor to ease the natural endorphine to bind on that.
Maybe its not enough to kill the cravings completly, but its useful. Where I live they start to sell cbd-based cannabis (the thc is illegal) with concentration up to 10-15% cbd and <0.5% thc (0.5 is the threshold to consider that weed legal or not), and i found to help a bit my cravings for opiates.
Im on bupe maintenance, 4mg daily, its enough to stay cool but sometimes i crave so bad for any kind of opiate. Using cbd it helps. Its only my opinion, im not a doctor nor a biochemist, but i think that cbd can be an option to ease the withdrawal (maybe in combo with other meds) or chill out during maintenance treatment to avoid relapses.
While the thc sometimes make me anxious or crave more for the DOC, cbd doesnt do that, it relax you without tripping.