There was an alien (mon calamari, I think) briefly shown in a bacta tank.

Best I could tell with Luthen is that he needed the external ventilator to keep breathing, which might not have been compatible with a bacta tank. Tanks are always shown with much simpler respirator setups. It may have been the plan to stabilize Luthen on the ventilator to a point where he could be transferred to a bacta tank.