…The resulting radius will always be a segment of a circle and is variable in size from the radius of the table saw blade downward …
a) Actually, the resulting figure will be a segment of an ellipse, the amount of eccentricity of which is dependent upon the angle to the blade. The steeper the angle, the more the resulting figure will approximate a circle, and the shallower the angle, the more eccentric the elliptical figure.
b) Geometrically, this is not true. The only circular arc you can get using this method is the arc of the diameter of the blade itself. All tighter arcs are elliptical. The tighter the radius you need, the more eccentric. This means that the closer the arc gets to the table top, the flatter it gets. The arc is tightest (smallest "radius") at the top, or in this case, in the center of the plane's cutter.
This is not a serioius problem, however. In fact it works in your favor, since it decreases the tendency for the corners of the plane blade to dig in, but it does mean that most of the cutting will be done at the center of the blade, especially for small spars. It also means you don't need several planes to shape a tapered spar. For making decorative cove molding, it is a totally insignificant effect.
A trick to minimize the effect is to use a 7 1/4" circular saw blade instead of the 10" table saw blade, or use a 6" dado blade. Don't use a whole stack of dado blades, because that will result in a flat at the top of the curve.
Yesterday, I got the spar I'm working on to a square state. Al least what is left of the timber I started with I can now lift by myself. Next Wednesday, I'll get it round, starting with the octagon gauge I made from your inspiration. I was just going to make it faceted, with sixteen sides, and then sand off the facets, but now I might try to speed things up a bit with a hollow molding plane, since I don't have time this week to hunt down the raw materials for, and build, a dedicated 3" diameter spar plane. I will file your exposition for future use, just as I have filed most of the others you have given us.
As an aside, I have another point of concern about the table saw method of making cove molding. It is not significant for one plane sole, but if you have many feet of molding to run, a table saw is probably not a good tool for doing it. Table saw trunnion bearings are not usually designed for the thrust loads generated by forcing wood sideways against the blade. This abuse can wear out the bearings prematurely. When I'm making molding, I usually hog out the majority of the waste with several cuts on the table saw, just to speed thinks up a bit, and then do the final shaping with hollows, rounds, etc., as needed.