My understanding is that if you don't have eligible points, but do have a blue card, you can book all with cash. However, I thought I remember reading that one person may need to be cash, but that could be for something different.
I agree...all you can do is call. I might call before it actually opens for booking and talk to the DVC cruise people...they might be able to let you know for sure so you don't wait on hold long that day only to find out you can't pay for everyone in cash.