I have completed every drop of content and gotten every companion and side character as a Sith Warrior; including Broonmark, Pierce, and I got both Quinn and Elara on Iokath.
However, when I booted up and said 'yes' to start Jedi Under Seige, 2V, Broonmark and Pierce all state 'they lost contact when you were frozen' to paraphrase, and I don't see Elara, and Quinn is simply unavailable. For record, I did side with the Republic on Iokath out of curiosity sake and for story interest.
UPDATE: Pierce is still on Odessen, and I believe is wearing the default outfit, Clickable and green, but nothing happens.
SECOND UPDATE: Pierce, 2V and Broonmark are now available, all listed twice now. I started on my ship and had the issue, traveling to Odessen fixed the issue in that, at least now, they are able to be summoned.
THIRD UPDATE: Sounds like someone said that gets fixed after the Ossus storyline.