Keep in mind, the guided devices with clamps have 'published' numbers for their angles, but more often than not, those published numbers won't be a true indicator of actual angles. Bigger & wider blades will often be done at lower angles than what's marked on the jig (sometimes much, much lower). The marked angles on the jigs are based on ONE theoretically 'typical' blade size, shape & profile, and clamped in a specific position on the blade. A larger knife, with a wider blade (greater distance from spine to edge) will likely produce a shallower angle, and a narrower blade will produce a steeper angle. Very long blades (such as the OP's mentioned knives at 18" & 20") will add variability to the angles, due to the widely-variable distance between the edge (at tip, mid-section and heel) and the top of the clamp, where the rod rides on it. In fact, those very long blades would likely have to be repositioned in the clamp, just to effectively reach all of the edge. In a nutshell, the actual angle produced by the jig would need to be accurately measured, after the blade is set up in the clamp, to have any real meaning.
Having put forth that long-winded explanation, I don't think a clamped/guided setup would work very well on such large blades (UNLESS, as originally asked, the guided system was proportionately scaled-up in size, for such large blades).
