I find it interesting that the OP has had essentially zero feedback on anything other than confirmation dialog boxes - especially since I don't think that was part of the OPs speculation.

Having some form of 'interactive spec' where stakeholders can 'play with' the system to verify intended behaviour is a really interesting idea. Of course it is used in software development all the time to varying levels of fidelity (wireframes, mockups, interactive prototypes and so on). But I think here is the rub... the better the quality the simulation the greater the amount of effort until it becomes approximately equal to the cost of just doing it.

Maybe with enough automation and tooling...