Buttermilk? Interesting idea. May I suggest another method? How about mixing some real-buttermilk powder (the stuff you get in cartons at the supermarket is not really buttermilk) into the dough? That would allow you to keep the hydration (amount of liquid) and buttermilk (amount of powder, not liquid), separate and give you much more control over your experiments. Here, in NYC, Saco Cultured Buttermilk Blend, very popular with bakers, is readily available in supermarkets.