The evolution of complex distributed software systems often requires intricate composition operations in order to adapt or add functionalities, react to unanticipated changes to security policies, or do performance improvements, which cannot be modularized in terms of existing services or components. They often need controlled access to selected parts of the implementation, e.g., to manage exceptional situations and crosscutting within services and their compositions. However, existing composition techniques typically support only interface-level (black-box) composition or arbitrary access to the implementation (gray-box or white-box composition). In this paper, we present a more structured approach to the composition of complex software systems that require invasive accesses. Concretely, we provide two contributions, we (i) present a small kernel composition language for structured gray-box composition with explicit control mechanisms and a corresponding aspect-based implementation; (ii) present and compare evolutions using this approach to gray-box composition in the context of two real-world software systems: benchmarking of grid algorithms with NASGrid and transactional replication with JBoss Cache.