Always center the location on the BC, and have this trigger for the reaver:
| Trigger |
| Conditions: |
| ¤ Always |
| Actions: |
¤ Move all Protoss Reaver owned by [Player] at 'Anywhere' to '[Location]'
|
| ¤ Preserve Trigger |
[Player] is the player that owns the reaver, and [Location] is the location centered on the Battlecruiser.
If the BC goes over cliff/water/doodads, the reaver will just stay where it is, and not move until it has ground to be on.

Undead-Fox beat me to it.