For PCF development, can you please provide a way to use the sdk WebApi and Utility.getEntityMetadata inside the harness. This could be some ideas for this purpose, but maybe you have some more:
Of course a "fake parser" based on some sample data would be best (something like FakeXrmEasy does for PlugIns (https://dynamicsvalue.com/home), because this way we could make also unitests.
There could be some other scenarios like have a connection to a CRM/CDS System somewhere.
Maybe the fastest workarround would be to allow us to know at least that the page is running inside harness (some property somewhere); this way we could provide some error handling with sample data. It's not a nice approach, but maybe this could be usefull also for other stuff too (like fake context)