You can use datasource property type in widget xml which will allow you to select the Microflow/Nanoflow/...
<property key="data" type="datasource" isList="true" required="false">
Not the most fancy solution, but since the pluggable widget API is indeed still missing this feature; Load the Mendix Client API within your pluggable widget. Use the mx.data.action API to call a microflow and the callback function it comes with to read the output of this microflow.
Another way of handling this situation is to use an attribute property. The microflow should set an attribute value of the context object and the widget responds accordingly.