We present an exploratory reflection on the role of pop-up participatory exhibitions in the design process of an upstream technology. To involve the public in the design of novel projected user interfaces for tabletops, we combined ideas from design fiction, participatory design, and user-centered design with a pop-up science shop concept. The resulting approach aims to engage a broad audience in ideation, user research, and outreach via an exhibition of functional prototypes in a highly-frequented location. In four one-week exhibitions, over 400 visitors were guided through the exhibition in small groups and involved in small design and evaluation activities. Through ethnographic field research we gathered insights from different perspectives, ideas for new use cases and applications, and feedback on the presented prototypes. In this paper, we describe concept, methods, and properties of pop-up participatory exhibitions and their relation to other participatory approaches. We share our experiences, discuss design choices, and provide practical suggestions for implementing such exhibitions.