Permatex has an exhaust sealant that they say is good for up to 2000 degrees. I just put it in my collectors. Seems to be holding up so far.
Clicky
The instructions are pretty vague. I read not to follow the directions on the package. It is a two step cure process. First, put it place with the pipes wet with water. Let this set for 36+ hours. Then run the car at idle for 15 minutes for the final cure. If you do part two before part one has had enough time to set, the exhaust gasses will just push the product out and make a mess.