Just checking to see if anyone has any code examples using GetParentRow(). I noticed the documentation does not have any examples for this or for ParentRow for the GridTableView class.

After playing around further I found some implementations that produce what looks like the expected output. Here they are for reference in the event that they may be useful to others. The $_row variable references a row at the third level of a three-level hierarchy.
$master_table0 = $_row->GetUniqueId();
$master_table1 = $_row->GetTableView()->GetParentRow()->GetUniqueId();
$master_table2 = $_row->GetTableView()->GetParentRow()->GetTableView()->GetParentRow()->GetUniqueId();
So the next question would be what can we do with the UniqueIds of these rows? (haven't found anything in the docs yet on what the UniqueId can be used for... it seems reasonable that it might be useful to pull data from specific grid rows?)