so I really hope that none of these 30ish zfs test failures I'm getting are due to my fix...

I'd be surprised. There are several that are either flaky or buggy

I can usually get down to 8 failures after a few repeated runs (using zfstest -l with the path to the previous run's log)

Here's my most recent set if it helps to cross reference illumos.org/issues/16423#note-4

→ FEATURE 16423: Import fletcher-4 algorithms from OpenZFS (Closed) | code.illumos.org/c/illumos-gate/+/3386