Coverage-guided fuzzing has been widely applied to address zero-day vulnerabilities in general-purpose software and operating systems. This approach relies on instrumenting the target code at compile time. However, applying it to industrial systems remains challenging, due to proprietary and closed-source compiler toolchains and lack of access to source code. FuzzBox addresses these limitations by integrating emulation with fuzzing: it dynamically instruments code during execution in a virtualized environment, for the injection of fuzz inputs, failure detection, and coverage analysis, without requiring source code recompilation and hardware-specific dependencies. We show the effectiveness of FuzzBox through experiments in the context of a proprietary MILS (Multiple Independent Levels of Security) hypervisor for industrial applications. Additionally, we analyze the applicability of FuzzBox across commercial IoT firmware, showcasing its broad portability.