Personally I would not. If you have a need for such a behavior, I suppose you could get the full list of free addresses, and then use your own algorithm to choose an address. However that would be quite inefficient.
Yes, anything in square brackets is an array. That section would look like this: "exclude": ["192.168.1.1","192.168.1.2","192.168.1.3"] You could also place an exclusion range or a set of reservations in the network, to block out addresses.