When can milk substitute for buttermilk in sourdough recipes?
I think your understanding here is incorrect.

The idea is to leave it overnight and let the sourdough SCOBY incorporate the buttermilk culture.

With a mature, functioning starter, there won't be much of any &quot;incorporation&quot; overnight, in the sense of actually reaching a new equilibrium of species in the starter. I don't think the buttermilk is being added to change your starter, it is just there because it is a common ingredient for pancakes and the like.
So, just go ahead and do whatever substitution you like that would be appropriate for the same batter without sourdough. You can expect analogous results, maybe even closer to the original, since you already have some lactic acid production from the sourdough.

curious about whether there's a flavor or safety reason for preferring prepared buttermilk

Flavor yes, buttermilk tastes differently from buttermilk substitutes. Safety no, adding buttermilk to a batter doesn't make it shelf stable.