Just been asked this in a private message. Public discussion spreads the load and brings better harvests, so here's a first seedling.
Code:
tell application id "OFOC"
tell default document
-- SOME FOLDER, LET'S JUST GRAB THE FIRST WE SEE ...
set oFolder to first folder
-- ALL ACTIVE CHILD-BEARING PROJECTS ENCLOSED BY THIS FOLDER, REGARDLESS OF NESTING
set refProjects to a reference to (flattened projects of oFolder where number of tasks > 0 and status is active)
-- HARVEST THE PROGENY THEREOF (PERHAPS FILTERED) AS A LIST OF LISTS
set lstTasks to flattened tasks of refProjects where completed is false
-- FLATTEN THE LIST, IF NEED BE
set lstTasks to my FlatList(lstTasks)
end tell
end tell
on FlatList(lst)
if class of lst is not list then
{lst}
else if lst ≠ {} then
FlatList(item 1 of lst) & (FlatList(rest of lst))
else
{}
end if
end FlatList