Never underestimate the power of the internet. A short while ago some kind anonymous soul (thank you!) noticed my write-up and some of the issues I had with the Chenbro case (specifically, the SATA hot-swap backplane). They let the manufacturer know, and Chenbro tech support posted a comment with a possible fix.
Their fix was to solder pin 11 to either pin 10 or pin 12 on the connectors to disable the staggered spin-up. Since my soldering iron is kinda chunky, and I didn't really want to do anything permanent to the boards in case this wasn't actually going to result in a fix, I busted out my multimeter and looked for alternatives.
Each board has two drive connectors... one of the pins for one of the connectors traces out to a jumper, but my luck ran out there -- the other traces out and terminates at some teensy-tiny little fleck of metal that would be just as hard to solder to. Strike one.
I tried Arctic Silver... which, as it turns out, doesn't conduct. Strike two.
Next I tried a loop of wire... and that seemed like it might work, but the positioning was tricky. In the end I used a piece of stripped conductor from a cat-5 cable (nice and thin gauge), bent it into a "U" shape, and pushed through the gap between the connector and the board, then twisted it together -- effectively grounding pin 11 to pin 12. I made sure to test that pins 14 & 9 weren't grounded, and that 10, 11, and 12 all were grounded.
I'll leave out all of the wrestling I did with the case here, but I will point out that prior to shoe-horning those two boards back into place, you should make triple sure that those little wire loops are firmly in place and that those pins will remain grounded no matter how much the board shakes.
I also ran into a problem where the drives would get power (green) but not data lights (blue). After a more testing, I narrowed the issue down to the fact that the back-plane boards seemed to flex just enough that my drives weren't seating all the way. I jammed a couple pieces of wood between the boards & the metal plate that carries them to reduce the flex and that seemed to help.
End result -- WOOT! All four of my drives are online, hot-swappable, and my ZFS pools are still rockin' away.