I'm glad to hear the code change worked for you and you were able to recover your parts!
I did some more testing on this and other software-only fixes, and while I was able to recover some parts in each case, I couldn't find a single workaround that could recover all parts. Some devices (the ATtiny85 in particular) are particularly stubborn and really need the faster risetime on RESET to recover from 0ms SUT fuse settings.
Rather than rely on a software-only fix that may or may not work reliably across the various supported devices, I have decided to rev the hardware. This will hopefully provide a more robust fix that works uniformly across all parts. As soon as I get some parts in, I'm testing the use of a separate switching circuit that will allow me to leave the DC-DC converter on full-time and get <20us risetimes on RESET when I flip the switch. This is option 3 above and the "best" solution from an electrical standpoint.
The results of my testing will go into rev 2.1 of the hardware. I still need to do a bunch of testing before I order PCBs, so it will be a couple weeks before I have new kits available. I'll post updates here and to the site.