I've been disappointed that all the sourdough biscuit recipes I found included baking powder. A search here, however, revealed David's attempts at an all-sourdough version (http://www.thefreshloaf.com/node/21967/sourdough-biscuits-trying-real-thing-take-2).
I keep 8-10 oz of starter at 100% hydration in small quarter containers in the fridge. Yesterday it had been 7 days since I fed my King Arthur New England starter. I divided into a 3 oz portion and a 5 oz portion. Both were fed 1:1:1, and left on the counter. After 5 hours, the 3 oz batch (now 9 oz) was returned to the fridge. I left the 15 oz batch on the counter overnight in a larger container. It was bubbling wildly this morning. I followed David's recipe with the following alterations:
1) All butter. I had no lard (rectified that at the grocery this afternoon, I'll try again with 50/50 lard/butter). I used a food processor to mix the chilled butter with the AP flour (KA bread flour in this instance), sugar and salt. I hand mixed in the starter, and just barely got it to hold together as per David's advice. After a 45 minute rest, I did the 4x stretch/fold/roll.
2) Nearly a 5-hour proof. They hadn't risen enough after 2.5 hours, so I went to the grocery store. When I came home, they were nearly doubled, and got to sit another 45 minutes while the oven warmed.
3) 19 minutes @ 425 versus 15 mins.
The biscuits are light, and perfectly sour with just a little butter (also great with honey). We'll be having them with spiral ham and Tillamook cheddar tonight.