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.
You could always use two splitters (one reverse one) and just connect standard Omnia power supply to one end and then split it up at the end before connecting to the router. Its called poe injector and then poe splitter. Something like that:
Since i didn’t need the switch, i ended up powering the omnia using a meanwell 12V DIN-RAIL PSU. Which is next to a 48-56V DIN-RAIL psu powering a 8x PoE at power injector powering 2 unifi-ac ap. Both PSU’s, the injector & the omnia are housed inside the central house power cabinet. The Omina is also hooked up to FTTH using a SFP WAN module.