Okay, with expert guidance from Chris, I managed to engineer a Java-free RUI application architecture. It's not exactly simple. In test mode, any REST request goes through an invisible proxy in the workbench. The request that your service sees is not what the proxy sees; the EGL JavaScript runtime actual wraps your request inside a larger request which is sent to the proxy. The proxy unbundles that and sends it to the target service. The proxy then takes the response, rebundles it and sends it back to the runtime.

So, if you're writing your own RPG version of the proxy, then you have to do all that bundling and unbundling. Then you have to invoke the target service. I, however, skipped that. Since I have control over all the pieces, I extracted the actual business logic and re-wrote the target service to call the business logic. Then I added code in the proxy to call the business logic directly.

It's not quite as complicated as it sounds and it works really well as a proof of concept. I can now say with certainty that it is entirely feasible to write a complete, Java-free EGL Rich UI application, using only RPG as the back end.

I'll clean up the code and export the project (including a savefile with the RPG code) in the next day or two.