We propose novel tool support for increasing testability of software systems. The support comprises stub software libraries for testing programs on platforms on which regular libraries may not be available. This can be encountered during outsourced software development (libraries unavailable to outsourcing provider), and development/migration in advance of vendor-provided libraries. Briefly, a stub library comprises a cached front-end component (including package headers) for integrating with the client program in standard fashion, with calls to the library either being served by locallycached answers in the front-end, or by remote invocations of a back-end on a platform supporting a live image of the library (if available). The ability to cache library request-response pairs implies the method can be used to inventory usage of libraries as a part of the standard inventory process in an outsourced porting/migration/development engagement. We discuss the compile-time and run-time issues raised by the tool support and the solutions we provide for them.