Reviving this thread.
I tried to use the family plan for the first time, to add additional rooms at Hyatt Indian Wells (former HGC) and Hyatt Huntington Beach later this month. The good news is that it's available and really is half off the prevailing rate, currently about $75 at Indian Wells and $100 at Huntington Beach (though no discount on the resort fees).
The bad news is that I'd already DSU'd those reservations, and they can't guarantee connecting rooms for a suite, or even adjacent rooms in the same part of the hotel. Thus I gave up on the idea.
So at least at those two properties the family plan and DSU are both great options, but hard to combine to produce a two bedroom suite. The agent wasn't really clear about whether those suite types actually don't connect or if they just don't want to allocate me to a specific connecting suite in case that blocks out something someone might pay for.