Why not just splice/split the ACC line to power on both the M2-ATX and Opus POL when the ACC (ignition) turns on?
I am not sure about the Opus but the M2-ATX has a 4 second delay to wait for the power to stabilise before it will send the turn-on pulse to the computer. By this time the USB hub should be all powered up by the Opus POL.
You should be able to provide 5v power to USB devices from one source (Opus) and have the data lines connected to the computer powered by another source (M2-ATX). That is definitely feasible.
But if your system is not that power-hungry, you can try to simplify things and have the M2-ATX power the hub from the 5v line as well.