A"Diamond Force" is an FT name for a "special arrangement" whereby the Diamond Desk makes special representations to a hotel which shows no award availability. Usually, the full cost of the stay is billed directly to the Diamond Desk.
The request is not always granted (depends upon the property, the time of year, the length of stay and the days of the stay). Some properties just do not participate.
As a preliminary, start searching from the number of days for a PAID stay for the months that you are interested. This will give you some idea of the demand. Also look at the room rates for the period that you are intersted - if they are much higher than the rates for the low season (you will have to research) then you may not be successful.
Good luck