> all you have to do to port code over is change the calls to file selection dialogs (which return a list of file names) to "powerbox" calls (which return handles),
Actually you don't even necessarily need that. Assuming you have operating-system-provided file selection dialogs, you can just return "/app/d41d8cd98f00b204e9800998ecf8427e/spool/whatever-the-fuck-it-is.ext", and only allow the app to read and write that directory. Although that's a bit more balky in general, it can handle most use cases without even needing to modify the offending program.
Actually you don't even necessarily need that. Assuming you have operating-system-provided file selection dialogs, you can just return "/app/d41d8cd98f00b204e9800998ecf8427e/spool/whatever-the-fuck-it-is.ext", and only allow the app to read and write that directory. Although that's a bit more balky in general, it can handle most use cases without even needing to modify the offending program.