Software engineers often use hand-drawn diagrams as preliminary design artefacts and as annotations during reviews. We describe the addition of sketching support to a domain-specific visual language meta-tool enabling a wide range of diagram-based design tools to leverage this human-centric interaction support. Our approach allows visual design tools generated from high-level specifications to incorporate a range of sketching-based functionality including both eager and lazy recognition, moving from sketch to formalized content and back, and using sketches for secondary annotation and collaborative design review. We illustrate the use of our sketching extension for an example domain-specific visual design tool and describe the architecture and implementation of the extension as a plug-in for our Eclipse-based meta-tool.