In theory, there is nothing that would prevent it working. But haven’t tested it myself. 3 A is IMO a little bit lower than the rating of the Omnia (3.33 A), but if you won’t utilize all USBs and connect a magnetic disk at the same time, you should be okay. I did some power budget computations earlier: Omnia +5V0 output max current - #2 by peci1 .
We’re running Omnia from a custom power source switched between AC and battery power and there haven’t been any problems so far, not even during the AC/battery switchover.