@SaperPL Do you have any 3D cad files, or even drawings with dimensions?
EDIT:
Thinking more about the HDPlex and full size GPU.
If you put the HDPlex on the right of PSU tabs against the wall (remove the USB front ports), and then mounted the AIO inverted with the hoses running underneath it with something like the kraken m22, you would have around 6-8mm of gap to run the PSU Cables through. It might be a little tight, but you would only need the GPU and Mobo cable at most. (letting the CPU cable fold flat across the HDPlex) If you setup the AIO to exhaust air, you could pull in cool air on the VRM and then out through the radiator. You would also get air flow from the GPU side since it shouldn't be blocked at least next the AIO. A custom made mother board PSU cable with a built in flat 90 degree angle and correct length would keep the fan / radiator from being blocked too much as well.
Granted, this is all hypothetical until I can get my hands on a case, or some drawings.
That being said, you would be losing the front USB ports and potentially the power button. A mini tactile button with custom button cover and rear mount would give a less satisfying click, but probably better than a flat mounted capacitive switch.