I managed to bodge a plugin based on nbgitpuller:
If you go to the app /urlparams and pass two query parameters paramsfile=config.json and params={"a":1,"b":false} it writes the file, then redirects you to urlpath
Example on mybinder:
Double escpaing the query params for BinderHub makes this difficult to hand-craft URLs, it would be nice if BinderHub could process just the parameters it recognises and pass the remainder unchanged to the app. nbgitpuller has the same problem.