Hi Simon,
Looks like you found a good solution.
FWIW the idiom which seems the most intuitive to me is that of
making a new row at the end of the children of a given row.
Code:
tell application id "OOut"
tell front document
set oRow to first selected row
-- new row at end of children
make new row at end of children of oRow
set expanded of oRow to true
end tell
end tell