Are there devices to moderate speed on mountain descents without braking?
There are two parts to your question, and two answers. The first part of your question is whether there are devices to moderate speed on descents. This is a common issue with tandem bikes (and occasionally with bikes designed for loaded touring). Many tandem-specific rear hubs have a threaded left side onto which can be mounted a "drag brake." Typically a drum brake, they can be adjusted for a slight amount of drag; the rim brakes are retained as the primary brake for stopping. The advantage of the drum brake is that it's away from the rim (since excessive heating at the rim can have dire consequences for tube or tire) and they have a high heat capacity. Although no longer manufactured, the venerable and venerated Arai Tandem Drum brake was an example of this. 

Your second question is about how one would estimate the drag demand on a bicycle. This is a well-understood, though perhaps less well-known, problem, and is discussed in Section 2 of Wilson and Papadopoulos' Bicycling Science. As you surmised, the potential energy component has to be offset by drag elsewhere: either by aero drag, braking, or rolling resistance. Terminal velocity is achieved at the point where the potential energy loss exactly balances the drag forces generated elsewhere. The coefficient of rolling resistance (Crr) happens to scale exactly like slope, so a change in slope of 1% is exactly like a change in the Crr of .01. Sadly, for this purpose, you won't much be able to rely on Crr for appreciable extra drag -- commonly, Crr ranges from about .004 to perhaps .01. Thus, although in a theoretical sense you must take rolling resistance into account, in a practical sense it dissipates too little energy to matter. Deflating tires to increase Crr on a twisty descent is not just insufficient, it's a bad idea. 

Aero drag is more easily manipulable but is also limited in efficacy. Drag area (typically denoted by the product of the frontal surface area, A, and the coefficient of aerodynamic drag, Cd) for a typical cyclist on a typical road bike will range from around .3 sq meters (roughly, 3 sq. ft) up to perhaps .5 sq meters or higher. Aerodynamic drag force varies with the square of airspeed (since the force varies with the square of airspeed, the power demanded to overcome that force varies with the cube of speed) so sitting up, unzipping a jacket, and widening your arms and legs can add some CdA; however, rarely more than ~ .2 sq meters. 

That leaves braking, i.e., the conversion of potential energy via kinetic means into heat, and the place for a drag brake as described above.