I don't see a way to use custom widgets with the ArcGIS online version of Web AppBuilder. Is the only option using the developer edition if you want to add custom widgets?
Yes, presently this is the only way to add third party custom widgets to a WAB app.
In the past, I have been able to publish a geoprocessing service and add it to my WAB map without using the developer edition. But, now I do not see how I can do that. I suppose that has changed.
... I take that back. It looks like if you were to publish your own geoprocessing task as a service that you can add it to your WAB map without using the developer edition. You will need to add a new widget, click on the Geoprocessing one and then you can insert the URL of your published service there
Retrieving data ...