Last year I stayed at the Cayman RC using a Pointsaver Award. I followed the directions and didn't have any issues. It is possible that even if Pointsaver awards are valid for a hotel, they may still not have any inventory for those awards for the dates you are looking at.

When I was looking for dates last year, I noticed that any dates that the RC had available for awards, they would take the PointSaver awards. I don't know if that was just luck on my part or normal though. But if that is the case, then you can make the reservation with RC and then call MR to order the PS award. MR should know what a PS award is and how to check whether it is valid for the dates you have reserved.