The growing availability of connectivity/computing/storage resources on smartphones and tablets together with the trend toward frequent and voluntary collaborations in user communities (such as in experience/status sharing and shopping recommending applications) are enabling novel scenarios of high relevance for the user mass market. In particular, we claim that the social-aware sharing of under-utilized resources in physical proximity will be of paramount importance in future heterogeneous wireless networks, i.e., spontaneous networks (SNs), where neighbors opportunistically and temporarily cooperate for service provisioning. In this paper we propose a novel middleware for multi-hop SNs, by specifically focusing on the hard technical challenges of supporting collaborative re-casting of live multimedia flows with dynamic quality adaptation. Our middleware prototype originally (1) adopts a middleware-level multimedia redistribution approach with cross-layer visibility of underlying SNs, (2) dynamically exploits collaborating nodes to monitor and tailor end-to-end streams by splitting them in sub-segments in a completely decentralized way, and (3) performs quality/resource-aware management decisions with limited resource consumption at collaborating peers. The reported experimental results demonstrate that, notwithstanding the middleware-layer approach and the challenging characteristics of multi-hop SNs, our solution effectively supports redistribution of tailored multimedia content with limited overhead.