I don't think you can use an xpath on a grid directly, but you can use a microflow to prepare the data and then use an intermediate associated object to show it, i.e. you don't have to use a microflow as the datagrid source and do the query every time.
The microflow would be like this:
https://modelshare.mendix.com/models/3af7965b-686f-4c8d-a1f0-c83698554261/casttoauthoranduseinxpath
Inside the expression:
First cast the $currentUser to the required Author type, then use it in the xpath.