Emulating API for consumers

We develop and manage a set of web services that in turn communicate with many third part systems. For our integration testing and UAT environments we have always built simple emulators (think wiremock, for all of the third party platforms where we can control the response based on input to cover expected success and failure scenarios. This means we have full control and have no reliance on the test systems of other companies etc.

Now that we have more users plugging into our apis (we also offer a gui as an entry point) we are getting more and more requests to change data in the test environment to artificially progress the status of some process or to force errors. As we get more and more users, we would have to spend more and more valuable time on supporting this. Ideally they would develop their own emulation of our system but many seem unable or resistant to do so.

I was thinking about developing in house a simple application for emulating our web services where the users could host it locally and control it as they need. Now i can see this being a pretty task, but overall its likely to be a time saver. I was wondering if anyone has any experience with this kind of approach and if there is any useful stuff out there for this kind of thing?

Our stuff is mainly rest apis build using jaxrs so stuff like swagger or wiremock could be used pretty easily.