[boromirofborg]

Can you hail a ship that's flying by because of Cytherians? (To ideally make them stop and waste part of a turn.

Actually, by re-reading, my guess is you can. And they do stop. And then continue moving again because they still have range and all you really did is waste a hail.

[BCSWowbagger]

Hail is one of the many Premiere cards that is not worded correctly. The Glossary explains what this card actually means:

Hail
Although the ship targeted by this interrupt is not "stopped" (e.g., it may initiate battle or attempt a mission), it may not move this turn. See passing locations.

In short, your trick works.