Glenn passed along this link to a paper on a new technique for researching end-user functions for complex applications. Their focus seems to be on adding functionality to an existing complex application, but many of their findings support the need for the likes of protokit to develop rich prototypes quickly.
Unlike classic Wizard of Oz, Champagne Prototyping gives the user access to a rich interactive environment that genuinely can be executed on the computer—but the core feature of interest is not actually executable by the computer or even by the researcher. The rich executable context allows the user to interact with the system in order to understand the circumstances in which the new feature would be applied and the effects it will have, but finesses the fact that the feature itself has not been implemented.
Read the Champagne Prototyping: A Research Technique for Early Evaluation of Complex End-User Programming Systems by Alan Blackwell, Margaret Burnett, Simon Peyton Jones for more details.