Hmm. Not sure about changing the properties of the fibers and thus the pfsConfigId for darks and biases. The configuration of the fibers is not changing.

Also, would you only change the status for science fibers when taking arcs/flats? Is UNILLUMINATED supposed to mean expected-to-be-dark, or something else?

I don't think the hash ever change if we update fiberStatus.
But your second point still hold, the configuration of the fibers effectively not changing.
I'm not sure it matters very much since fiberStatus is not used anywhere for biases and darks reductions, but I'll defer to rhl
I will change the status for science fibers and engineering fibers for arc, flat and even object because technically nothing forbid to illuminate the engineering fibers while exposing on the sky.

UNILLUMINATED = 5, "not illuminated; ignore any flux"

Yes, "expected-to-be-dark" is my current understanding.

merged as spsActor 1.6.7, can be deactivated through actorConfig
I've let biases and darks untouched for now.