Great idea, Robin- but the Julia child recipe cooks for over 3 hours with the meat in it, the seared beef tenderloin would be at high temp, then have to be refrigerated for hours awaiting the sauce to be safe. I think it'd be hard to get it back to temp without it overcooking in the sauce.
I'm thinking of doing the same thing but with beef bones- I roasted them as though for brown stock, then I'm going to add the rest of the ingredients except the beef and cook for a while. That'll get me the caramelized meat flavor in there. Then separately pan sear the tenderloin cubes when everything else is ready. We'll see what I decide: I'm still liking this idea better than the Pepin recipe idea and I'd already browned the bones before realizing I had the wrong meat..