@Josh | NFC I took some photos of the inside of my S4 and did a little test fit proving that, at the very least, it would be possible to reverse the PCIe slots in the S4M and mount a Zotac 1080mini / 1080ti mini in this alternate orientation without expanding the case. This may or may not hold for other GPUs.
I'm going to see what I can do about creating a model for the hypothetical alternate part because these pictures and drawings aren't great.
EDIT: Here's a link to an early sketchup model for the hypothetical reverse GPU chassis back / bottom. Since the sketchup model I was working off of doesn't appear to reflect the S4M 100% accurately, this isn't likely a viable model for a real part. Any chance there's a model of the S4M floating around the internet that is 100% reflective of the final S4M design? Either way, I'm going to convert this sketchup model to solidworks and get some quotes to have a one-off part manufactured.
This configuration would not be viable for brickless builds, GPUs with power connectors on the top of the card that are too close to the motherboard, or builds with sata drives. A small amount of space above the GPU would likely have to be sacrificed to ensure there is enough room for the PCIe riser cable underneath the GPU, but the configuration should still allow for full thickness fans to be placed on the GPU heatsink.
Waiting for my parts to get in on Monday so I can do some benchmarks to show why / if anyone should care about the possibility of mounting their GPU "upside down."
SECOND EDIT: Riser cable and A9s showed up today. Riser cable was a bust, will need to try a different cable. A9s seem to be a marked improvement over the A9x14s, but I didn't do any proper benchmarks with the A9x14 without the side panel on, so it's hard to say exactly how much of an improvement the A9s made, versus how much of an improvement pulling the side panel off the case made. Seemed to be about a 7C improvement at least, but the card was maxing out around its thermal limit with the A9x14, so the difference might have even been a little bigger, had I been running the card in its purely stock configuration.
Heard back from one sheet metal fabricator, who said that they cannot make the part using their conventional press brakes. Anyone have recommendation for good shops that I might want to reach out to who might be able to manufacture this part?