You can use the isPath and pathType properties for this (both valid for entity and attribute properties). Note that currently only outgoing associations are selectable. pathType is either reference or referenceSet, isPath can be yes, no or optional.
<property key="ownerattribute" type="entity" entityProperty="entity" isPath="yes" pathType="reference" required="true">
Pieter, concerning your previous comment,
you can use the functions initContext(), applyContext(), object.getAttribute() (returns the GUIDs of the referenced objects) and mx.processor.getObjectsById. (we are working on documenting these functions)
The image viewer, inline object creator, dynamic microflow trigger and explorer pane widgets for example use those functions. You might inspect their source by extracting the .mpk files (which are found in the widgets directory).
For retrieving the Dataview context, I suggest you take a look here: Using the Dataview context object.
You can retrieve a reference using:
Where assocPath is a string like "System.User_Image", it will return the GUID of the referenced object, which you can then retrieve using a simple getObject:
mx.processor.getObjectsById(["" + assocID], dojo.hitch(this, this.showButton));
Good example of this is in the DynamicMicroflowTrigger, which uses this exact same method.
To add a reference, you can use this:
First parameter is the association, second part is the GUID.