In digital music-making activities musicians with physical disabilities employ both accessible and generic control interfaces; accessible controllers capture broad input gestures and map them onto discrete output events, whereas consumer digital musical instruments (DMI's) offer extended control only through artefact multiplication (more buttons, sliders and dials). The interaction paradigm common to both consumer and specialised controllers reveals limited dimensions: click-and-drag or select-and-move. It is common practice in inclusive music activities for an able-bodied facilitator to expose access to low-level parameters via sequential and non-real-time processes, on behalf of the musician with a physical disability; these factors constrain real-time independent creative self-expression. The current research details the explicit needs and capabilities of a small group of digital musicians with quadriplegic cerebral palsy, garnered through participatory design and mixed methods. The project methodology draws on tools and models from the fields of assistive technologies and from mainstream DMI design. Project participants contribute data pertaining to preferences and capabilities, and evaluate key iterations of the evolving prototype. The practice-led and participatory design ethos relies on demonstrably repeatable and preferred gestural capabilities, without seeking to maximise physical ability in a rehabilitative context. The underlying mapping strategy exposes, in real time, a transparent hierarchy of dynamic sound parameters commonly accessed through facilitated, offline and sequential processes.