Large-scale distributed real-time systems are increasingly difficult to analyze within the Rate Monotonic Analysis framework. This is due partly to their heterogeneity, complex interaction between components, and variety of scheduling and queuing policies present in the system. In this paper we present a methodology to extend the traditional RMA approach by allowing general characterization of workload and flexible modeling of resources. We realize our approaches within ProtEx, a toolkit for the prototyping and schedulability analysis of distributed real-time systems. This toolkit focuses on a wider set of methodologies than the traditional RMA scheduling analysis tools.