There's another way to keep the meetings secret and stay within the 15 minute criteria. At our workplace we have to schedule meetings ahead of time far in advance but often because of the nature of our work they can get cancelled at little or no notice. Our way around that is to schedule for more than one day the same meeting, and the first time we get to meet is the 'meeting', the rest are then just cancelled.
So all the DHS needs to do is schedule lots and lots of meetings, give the required 15 days notice, and hey if the meeting gets cancelled then so be it. But then because of the scheduling and the publication of these meetings - it's in the public record that they're meeting - they're just *always* in meetings, that happen to be always closed.