If money is no object, why not have a frame builder make a steel combo bar/stem fitted for you? A cromoly steel handlebar will be less noodly than an untreated 6061 or 2041 aluminum bar.

What handlebar do you want to use? If it's 31.8 clamp dia, then your only option is going to be a threadless stem and an adaptor.
The connection between the stem adaptor and the steerer tube will probably have the same stiffness as a quill stem. The flexibility, or lack of stiffness, you perceive may have more to do with the handlebar and bar to stem connection.

Another valid question to as: is why do you require a 'stiffer' bar/stem? Are you competing? Or, do you truly like and want a threadless stem solely for the convenience of changing bars often, and aesthetics?