H2O-Micro uses only a 19V 350W Adapter, the HP one. It is connected to the motherboard power jack and I've connected the HD-Plex internally via the 2-pin Mini-Fit Jr. connector.
I've done extensive modding to the HD-Plex because of size constraints(All connectors soldered off, but what I've done in order to get it working is short the PS_ON signal to ground using a MOSFET, the gate of the MOSFET is pulled up by using the 5V from the SATA power connector.
So just mod or make your own 24-pin jumper, but use a MOSFET for switching.
I used the 12V first, but that seemed to cause issues, probably because the rise time of the 12V is longer then the 5V, I couldn't get the system to boot from standby because if this.
Also keep in mind the PCIe socket power limit, Thin ITX is restricted to 25W, most GPUs will work fine with it, but I made sure that I supply ALL 12V from the HD-Plex.
And lastly the Gigabyte boards have open ended 4x slots, allowing you to use a 16x tot 16x risers, the ASUS Q87T is not openended, I cut it away in my tests o make it open, but there are capacitors behind the slot that interfere with 16x cards.
Replace the +12V with the +5V from the Sata connector on the motherboard, Most Thin ITX motherboard have a Sata Power cable to go with it and a slimline power connector at the end.
