Yes you can make stock from those bones too.

I like to save onion tops, carrot peelings and celery ends for my stock, too (even mushroom ends. especially for vegetable stock). Throughout the week I just put them in a container in my fridge until it's stock time! I brown the bones with the vegetables and whole peppercorns. Makes great stock!

Another yes to your question. There is still flavor in the bones, although not as strong as if they were raw. I would use the same method as darksideofthespoon recommends. Consider "painting" the bones with tomato paste before browning with the aromatics as it will give your stock a nicer color.

I love all these ideas...and short ribs for stock? Delicious! Fat cells are made in the bone marrow. I'm a huge advocate of skimming the fat off as the stock simmers or your digestive system will pay for it. Freeze your stock in 16 oz containers for greatest versatility (could also remove fat after freezing). Homemade stock has lots more calories than other stocks, too.

I completely agree with the "painting" the bones with tomato paste before roasting. That's what I do with oxtails for a great stock. If using oxtails, you need to strain the stock, chill it then remove the far. Oh so much flavor!

Butchers who buy whole sides of beef and/or kill their own cattle usually sell bones for the best price. I like to paint my bones with red miso paste for extra umami.

I'd recommend using those bones as well as uncooked bones. You likely will get a weak flavoured stock using only the cooked ones