Drawing upon multiple disciplines, avant-garde fashion-tech teams push the boundaries between fashion and technology. Many are well-trained in envisioning aesthetic qualities of garments, but few have formal training on designing and fabricating technologies themselves. We introduce Mannequette, a prototyping tool for fashion-tech garments that enables teams to experiment with interactive technologies at early stages of their design processes. Mannequette provides an abstraction of light-based outputs and sensor-based inputs for garments through a DJ mixer- like interface that allows for dynamic changes and recording/playback of visual effects. The base of Mannequette can also be incorporated into the final garment, where it is then connected to the final components. We conducted an 8-week deployment study with eight design teams who created new garments for a runway show. Our results revealed Mannequette allowed teams to repeatedly consider new design and technical options early in their creative processes, and to communicate more effectively across disciplinary backgrounds.