You wouldn't be crazy at all... it's called a reverse sear... very popular cooking method.
I completely agree with your thoughts about PBC tri-tip... it's "ok", but really needs a good sear/crust added to be GOOD.
Another easy method would be to pull your TT at 120, set aside for 5 minutes, keep the lid off the PBC and return the TT when the fire is raging. The PBC can lay down a nice sear when the fire is allowed unlimited oxygen