Progress!
Not much time recently, so not that much, but ...
The shitty PSU is now a shitty semi-modular PSU. Great success!
Cut (and insulated the ODD power cable too - it was just annoying.
I also made a mess cable harness.
There was absolutely no way to get the insulators of both leads into the pin, so I stripped them extra far back, twisted the leads together, crimped, and double-heatshrinked them for stress relief and insulation. From giving them some pretty serious yanks, this should hold. It's also worth noting that the later wires turned out a lot better-looking than the first one that I took a photo of. My brother (who is an electrical engineer) will probably have an aneurysm if he sees this, though. Ideally, I would solder the ends, but my soldering iron is utter garbage.
I also got a riser cable. Of course, this was ordered when I thought I was going to mount the GPU fan-down. I should really get a shorter one. This one is 35cm (I wanted one that could potentially wrap around an ITX motherboard for a later upgrade). I'm thinking 5cm could be enough, but the only 5cm ones I can find are rather expensive (yeah, $30 is expensive for this build).
It fits, though! Barely. As for clearance - the white latch on the riser blocks the CPU fan, but only when it's open - if I close it, it's fine. I planned that! Definitely on purpose, and not dumb luck, not at all.
I made a GPU retention bracket from some of the metal cut out of the back of the case, some m3 screws and matching nuts. Works surprisingly well.
I should probably fasten it to the case too, though, as for now it's only resting against it. I have more screws and nuts, so I'll likely just screw the bracket through the case mesh.
Oh, btw, the PC has a front panel connector that gives off an error at POST time and refuses to continue without intervention if it's disconnected. It has four USB 2.0 connectors as well as audio, four debug LEDs and the HDD LED. Experimented a bit with unpinning parts, and it doesn't mind if the USB is gone. The rest got wrapped up in kapton tape and tucked away. Who knows, debug LEDs might end up being useful.
Lastly, I think I mentioned I ordered a low-profile USB 3.0 card for this - but in the excitement over finding the one model available with both internal and external ports, I didn't check if it came with a low profile bracket. I'm smart!
Luckily, I found a bracket that looked like it'd fit for $1 on Aliexpress.
And it fits!
Sort of.
Zip-ties to the rescue! At least the style of this build is consistent.
All in all, this is getting very close to done. Just need to finish the wire harness (still missing SATA and 4-pin molex connectors) and cut vents in the side panel. I've got an idea for a cut-out pattern that will both maintain the sleeper look somewhat, maintain structural integrity and be easily covered with cheap mesh. No time for that now, though, but it's coming up 
