This paper presents a middleware platform for the provision of services in disconnected MANETs, focusing on service invocation. This middleware exploits content-based communications (through a publish/subscribe paradigm) and employs a store-carry-and-forward approach for the network-wide opportunistic dissemination of messages. Service invocation is implemented on top of these communication features. The paper first describes the main aspects of the middleware and then details the service invocation mechanism. We namely show that the potential multiplicity of service providers can be exploited in order to increase the client satisfaction and that several network healing techniques allow the reduction of the network load. The paper ends by a description of some simulation experiments in a realistic scenario, whose results reflects the performance of the approach in terms of client satisfaction and network load.