yes, the ACC line has no power when the key is in the "start" position (engine crank). but that shouldn't matter because your engine will be started [and the key will be back in the "run" position] before your computer boots up.
the HDz300's power/ignition/ground wires can be wired normally (that is how I have mine hooked up and it works fine for me). if your power supply has a +12V output, then you could always wire the HDz300's IGN wire to that instead. this way it will have power as long as the PC is on.