Bread recipe calls for dry milk powder. Can I use dry buttermilk powder?
You'll be fine doing a straight substitution.  Well, maybe I shouldn't be so definitive since these are powders we're talking about and it's not quite the same thing, but I've substituted buttermilk for whole milk in bread recipes plenty of times.  The only thing that changed for me was the rising time.  When I use buttermilk in my sandwich bread, it needs an extra 20 minutes or so on the first rise compared to the whole milk version.