(first off at least, seeing what it looks like with all of them as subtasks of this one, but with additional links between some of those subtasks)

IT DID!
xref https://phabricator.wikimedia.org/T388022#11784455

apparently not with conduit either, at least not with maniphest.edit:

[{"type":"parent","value":"PHID-TASK-rdnghrhopik2clze6aiv"},{"type":"title","value":"test subtask creation of a hidden task that i can't see but that i know the PHID of"}]

or something like that i guess

wait what about conduit

turns out that they apparently can’t:

Parent task identifier "276" does not identify a visible task.

True, though that wouldn't restrict malicious actors from being able to edit via the API if they wanted to (as documented e.g. at https://we.phorge.it/book/phorge/article/forms/#use-case-security-issues:~:text=regardless%20of,Conduit%20API).