How to adjust an oven to accommodate two items with different cook times and temperatures?
I would cook the Brie at that same temperature for 30-35 minutes and it should be okay.

Alternatively, you could cook the roast fully, remove it from the oven, tent it with foil and then cook the Brie.  The meat can rest while the Brie is cooking and being eaten.  Prior to slicing the roast, heat up the pan drippings to a sizzling temp and baste to ensure a crispy crust.  But if the appetizer course happens to last for 20-30 minutes, then the roast will have been resting for 40-50 minutes and could lose too much internal heat.