. . . How about my idea of using twin parallel 'fences' in lieu of a mitre track?
As your mother used to say, "It depends."
That is, it depends on what is being done, and how the stock is being held. If the secondary parallel guide is used with a sled type of jig that holds the stock independent of the guide, it will act much like a feather board that has no "give" - except that the feather-board substitute is guiding the jig, not the stock.
Obviously, you'd want to ensure that the guide edge is straight, and clamped such that there is no binding of the sled through the range of travel. A well-jointed edge (smooth, straight, and square) that is waxed to reduce friction while keeping tolerances reasonably tight, should be OK.
What you'd want to avoid are situations in which the secondary guide could pinch or mis-guide the stock behind the cut. For example, the reason one shouldn't combine a miter gauge and a fence on a table saw is that as the stock is moved through the cut, any lack of precision in parallel will shift the vector of the cut in relation to the blade. That can result in the already-cut section of the stock binding on the rising teeth on the back side of the blade, causing a kick-back. Kazing, board through chest, or board through skull, are among the options.