Well, got it back to working after whatever happened last night. I think my poking around toasted the buck chip - well, not completely toasted. Screwed up the current trip level detection I think, because it would start but as soon as it tried to mine and detected an appreciable load the buck would reset.
So now I'm back to functional. However, the bigger problem I thought I solved is now a problem again and it's making me question how it was solved yesterday. I might have time to get back to it today but no guarantees. Hopefully I get it ironed out because I was really looking forward to using that power circuit on some other stuff for you guys.

If anyone's wondering, I'm using a pulled TPS53355 to generate about 2.6V across four nodes of BM1384. The '53 has an internal 5V LDO which drives internal switching circuitry and powers the switching FET gate drives. When the load increases, this 5V starts to sag. It's not a problem on any of the commercial miners I've looked at, and I've replicated some of those circuits in test but it still misbehaves on mine. I noticed some inordinate gate drive currents dragging it down, so a resistor inline with the boost capacitor solved some of that. Yesterday it started working beautifully after increasing the boost capacitor and 5V bypass cap, but now with the same parts in place it's still sagging so I'm a bit perplexed. Anyway it's running right now at about 175MHz because if I push it farther the 5V sags below 4V and the buck resets. I'm sure it's something very trivial that I've overlooked.