If I configure the job to trigger remotely and specify a token, I can trigger a folder computation successfully from a POST request even though the setting does not show up in the UI.
However, this setting does not propagate to any repository folders that the plugin picks up and creates, so you cannot remotely trigger those with a POST request. This means that you have to use polling, which is not desirable.
If I edit config.xml for the repository folder on the server directly and add the following block at the top of the file...
...then I can trigger the build remotely. This setting will get erased when the top level folder computation runs, so I can't recommend using it as a workaround.