It's not really a gray area. I was initially told how to do this by a DOT officer and had it confirmed by two others. It was later confirmed many times over at scale houses across this fine country of ours.
Back in the days of yore when I was at Con-Way NOW, I ran a lot of HAZMAT. So much so that there were some periods, sometimes for 2 or 3 months at a time, where I'd haul HAZMAT more than twice in a 7 day period and I'd just be logging every day as if I were driving a straight truck. But once 7 days had finally passed since my last HAZMAT load, I could log the previous 7 days as OOS when I got the next load that required logging.
If you haven't operated a CMV in the last 7 days, you are technically and legally OOS for those 7 days, and that's precisely how they should be logged when you begin logging in a CMV again. And of course, the instant you affix placards to a cargo van (or even a Prius), the vehicle becomes a CMV.
If you occasionally run a HAZMAT load in a van, like once or twice a quarter, it's really the next 6 days after the HAZMAT load that you have to keep track of, just in case you get another one within those 7 days. If you know you won't get another one within 7 days, or will turn one down if offered, then just log the HAZMAT load correctly and then forget about logging the next 6 days. Then a month or two later if you get another one, log the previous 7 days as OOS and log the CMV load like normal.
The on again, off again logging of HAZMAT in a cargo van is why DOT officers at scale houses practically drool when a placarded van pulls into one, because they know the chances of finding a log violation are very good due to the inexperience of van drivers with logs. It's the main reason that Panther won't put placarded loads on a van, for example. The biggest thing is to have those previous 7 days logged correctly, whether it's as OOS or actual hours, and that no days show HOS violations.