Everything in a beef stock seems to be about going slow and taking time. When it comes to roasting the bones though, it seems that most people recommend roasting at a really high heat. My instinct would be to roast them at a lower temperature to prevent burning. Is the higher temperature just to save time, or is there another reason?
This idea is to use the Maillard reaction (the reaction between amino acids and sugars as meats are heated and browned) to enhance and concentrate flavors. The reaction begins at about 280 degrees Fahrenheit, and tops out at 330 degrees Fahrenheit, so, you could use a lower oven. It would take longer. Caramelization flavors are more pronounced at higher temperatures. As long as your bones are well-browned, you will be in good shape. A lower temperature doesn't necessarily prevent burning, keeping any eye on your product is a better strategy.