I recently made a beautiful loaf of whole grain bread using the following method:
After transferring a toasted oatmeal stout from my primary fermenter to my secondary, I let the trub settle again. I carefully skimmed the rest of the beer off of the trub. I mixed 125g of this beer with 125g of bread flour. Usually with my sourdough starter, it takes 4-5 hours for this to become active. With the beer mixture, it took about half the time (2.5 hours). It was incredibly active. I then mixed together 187.5 g white bread flour, 187.5 g whole wheat flour, 7.5 g of salt. I added the 250g of starter and another 150g of water. I kneaded for 15 minutes.
This loaf proofed much faster than a sourdough for me. After 5 hours proofing in the fridge, I knocked back the dough and started a second rise at room temperature for 3 hours. I scattered with oats, scored, and baked in a high oven with steam for about an hour.
The resulting loaf was light for a whole grain loaf, with a caramel colour. It had a malty delicious aroma and flavour. A hint of licorice to it, from the oatmeal stout, which happened to have a high ABV. The beer had a low IBU rating, and couldn't pick up any bitter notes in the loaf.
The proof was quicker/more successful than I was expecting - probably because I pulled the yeast in between primary and secondary while it was still active, not from the trub after brewing was complete.
Definitely going to be experimenting further.