Hi Brian. Only the fields you define on your page type are shown in the fields list of the alternative form so you can't select the document name that way. However, I managed to figure this out so for the benefit of others who may have the same query this is how I solved it:
1). Edit your page type, switch to the Alternative Forms tab and create a new form. This must be called "Filter" to be able to be used on the Listing section in the Pages app.
2). Go to the Fields tab and add a new field to be used for filtering by document name. Leave the data type as "Text", tick the Display field in the editing form checkbox and change the form control to "Text filter".
3). Go to the Editing control settings section and enter the following in the Where condition format field:
DocumentName {2} {1}
Now, when you go to the Listing section and select your page type from the drop down list you will see the document name filter as per the default filters.