Okay, so I finally got a chance to try the riser out and decided to use it in an orientation where the GPU cooler faces upward, pointing in the same direction as the CPU cooler. See
this S3 mini picture for reference.
The S3 build uses a different riser- 3M I believe. But given the feedback I read of the HDPLEX riser, I expected the shielding as described by
@iFreilicht to be adequate to orient the card so that the exposed backside of the PCB will not cause interference or short-circuiting. But the card will not work when using the riser. The fan doesn't spin, and there is no video output.
I haven't seen actual case uses of this orientation with the HDPLEX. My guess is that the GPU can't be resting right on top of the cable. I also have tested it with the GPU oriented vertically, 90 degrees to the cable, but it still doesn't run either.
I'm not sure if the cable I got is faulty (it was bought used) or I didn't check the slot connections well enough. I'll give it another go later today.