Two-thirds of the world's population is expected to reside in metropolitan cities by 2050, placing an unprecedented burden on energy efficiency, environmental protection, and sustainable transportation. Southern European urban agglomerations, such as Naples (Italy), face specific challenges: a highly car-dependent culture, chaotic traffic, high air pollution, and a fragmented public transportation network. In this, Mobility-as-a-Service (MaaS) promises to integrate heterogeneous modes into subscription bundles, promoting a switch to low-emission, low-resource transportation. Understanding how people perceive and set up MaaS bundles is crucial for assessing their environmental and operational feasibility. I designed a novel iOS-based tool for conducting dynamic stated preference surveys of MaaS, the first of its kind in the literature. Compared to conventional questionnaires, the app collects extensive travel diaries and socio-demographic details. It interactively guides respondents to customize MaaS packages, reconfiguring sequences of trips and budgets in real-time. A case study in Naples explores how the platform can capture behaviourally driven responses in a car-dependent, environmentally stressed metropolitan area. Initial survey results indicate the tool's capacity to generate fine-grained microdata on adoption propensity and sensitivity to service features. Data enable the estimation of MaaS take-up scenarios and related implications for energy consumption and emissions savings. The described methodology indicates how dynamic SP tools have the potential to support evidence-based design of MaaS schemes in complex cities. The Naples case offers lessons applicable elsewhere regarding how evidence-based approaches can inform sustainable, energy-efficient urban mobility policies in cities with challenging transportation and environmental conditions.