Sadly, no. The issue is at a level below backups. And for a reason I haven't looked into yet, there are no previous NixOS generations available to roll back to.
The principal issue is that while booting, NixOS Stage 1 prints the message "cannot import 'rpool': more than one matching pool". The root filesystem is in that pool, so booting fails.
Unfortunately I don't know why it says that. From within a rescue system, I can see just one pool named "rpool", and I can import it and mount filesystems from it just fine. So the current task is figuring out why NixOS Stage 1 sees otherwise.