Right....the surface of the meat below the chicken will be at smoker temp.
OF course, I guess you couldtheorize that if bird drippings soaked into the meat BEFORE the brisket surface got up to temp (like right when you first throw the meat on) then its theoretically possible that contaminated drippings have touched the top of your beef.
However, we all know that a few drops of liquid aren't somehow making it to the middle of your beef cut. I mean, think about marinating overnight, or even for 16 hours or so...and how that marinade BARELY makes it past the surface of the meat!
I'd say if you had 1,000 people put a bird in a smoker that's up to temp, on top of a roast or brisket - 1,000 meals come out fine and no one is sick.
That's just my opinion!! It is based on logic, not 100% fool-proof scientific fact. But I have other things to worry about more than drippings.