This paper presents our on-going work on developing a flexible framework for formal co-simulation of human-centred cyber-physical systems. The framework builds on and extends an existing prototyping toolkit, adding novel functionalities for automatic generation of user interface prototypes equipped with a standard FMI-2 co-simulation interface. The framework is developed in JavaScript, and uses a flexible templating mechanism for converting stand-alone device prototypes into Functional Mockup Units (FMUs) capable of exchanging commands and data with any FMI-compliant co-simulation engine. Two concrete examples are presented to demonstrate the capabilities of the framework.