Here is a scenario:
An Imperial Lunar class cruiser (HP8) moves into base contact with a Tau Merchant class cruiser (HP6). The Imperial player declares a boarding action, so the Lunar may not shoot or launch ordnance. During the shooting phase, other ships in the Imperial fleet fire on the Merchant, lowering its shields.
Now it's the End Phase. Boarding actions are specified to be BEFORE damage control or removing blast markers. Teleport attacks have no specified time in the end phase, so the Imperial player resolves the Boarding Action first, doing one damage to the Tau ship.
So:
- The Merchant's shields are down.
- The Lunar is within 10cm.
- The Lunar has more HP than the Merchant, and is not an escort.
- The Lunar is not on special orders or crippled.
- The Lunar has not made any other Teleport Attacks.
Because Boarding Actions specify shooting and ordnance, but not Teleport Attacks, I'd say the Lunar is entitled to take a Teleport Attack against the Merchant.