Currently, the only way to access the majority of the FME Server REST API functionality via Workbench is to use the HTTPCaller.
The problem with this approach is that you need to hard-code the server URL into the HTTPCaller (or use a published parameter) but this is not dynamic between environments. If I want to publish a workspace from DEV to PROD, I need to go in and update the server URL.
It would be nice if a named FME Server Web Connection (which already includes the FME Server URL) could be used for both authentication and base URL configuration.
A simple wrapper around the existing HTTPCaller would probably be all that is required, so that the base request URL is automatically supplied from the web connection.
If the above is too difficult, please create a transformer to allow for the web connection URL to be queried in the workspace, so that this can be passed in to the HTTPCaller.
The ultimate solution would be to have a full FMEServer_API_Caller transformer that would allow REST API queries to be dynamically built within the transformer... but that's probably asking too much. :)