Due to the energy and resource constraints of the embedded Wireless Sensor Networking (WSN) devices that form Low-Power and Lossy Networks (LLNs), efficient support for mobile data sinks is essential. Further, such support should be fully compatible with the broad class of networking mechanisms now defined for LLN support, such as the routing protocol RPL. This paper describes the Hierarchical network of Observable devices with Itinerant Sinks Transporting data (HOIST) protocol. HOIST is designed to service LLNs using an RPL-friendly three-tier hybrid cluster-tree architecture to support sensor nodes deployed over geographically segregated areas. The mobile aspects of HOIST allow for collection from such remote fields without the need for longdistance wireless transfers, which would result in a higher level of energy drain. HOIST supports both real-time data capture and collected data retrieval. We have implemented and field-tested HOIST in a working physical system, as well as in simulation, with results indicating viability for both sparse and dense deployments of nodes under multiple, geographically constrained, uncontrolled mobility models.