ProsperOps uses regional RIs which don't include capacity reservations. We favor regional RIs over zonal RIs because they support instance size flexibility (ISF), which improves savings for Linux/Unix shared tenancy customers. In our experience, customers favor maximizing savings over reserving capacity.
AWS has released a capacity reservation feature called On-Demand Capacity Reservations which is decoupled from RIs. We're planning to add fully automated support for these reservations, which provide the best of both worlds: ISF savings + capacity reservation. Please submit feedback to register your interest.