I just found an easy way to get back to the master document.
If your occurrence is a FamilyOfPart member, then you can access its PartDocument and get the first CopiedPart feature from the DesignEdgebarFeatures collection. It's typically the 4th item.
This CopiedPart object has a FamilyOfPartsMember and a FileName property which return the currently used member name and the full path to the master document.
' ASM: Show the member and master of any FamilyOfParts occurrence in this assembly
Dim objAsmDoc As AssemblyDocument = objApp.ActiveDocument
For Each objOcc As Occurrence In objAsmDoc.Occurrences
Debug.WriteLine("* Occurrence: " & objOcc.Name)
If objOcc.IsFamilyOfParts Then
Dim objParDoc As PartDocument = objOcc.OccurrenceDocument
Dim objPartCopy As CopiedPart = objParDoc.DesignEdgebarFeatures.Item(4)
Debug.WriteLine(" - FamilyMember: " & objPartCopy.FamilyOfPartsMember)
Debug.WriteLine(" - MasterDocument: " & objPartCopy.FileName)