Hi Ian,
As mentioned by Freek retrieving a list of objects is currently not possible. I expect that this will be added to the pluggable widgets framework quite soon though, as during one of the latest meetups Danny Roest mentioned this to be on the short-term roadmap.
In the documentation it also says ‘Planned’ (https://docs.mendix.com/apidocs-mxsdk/apidocs/pluggable-widgets). So hopefully this will come available in one of the next Mendix releases.
Hope this helps!