Jean, thanks for setting me on the right path (no pun intended) . . . I checked out Curvy, which looks great, but then I remembered that Vectrosity -- which is what I'm using to generate the actual spline -- also comes with a spline-follow script, so rexamined that. And guess what -- it explicitly moves objects at a constant speed.
The best part? It's just a few lines of code:
do{
for (var dist = 0.0; dist < 1.0; dist += Time.deltaTime*speed) {
obj.position = myVectorLine.GetPoint3D01 (dist);
yield;
}
} while (loop);
Since that original block appears in the same file as the spline generation code, I pulled it out into a new script that I could attach to the prefabs that move along the spline, which required a slight modification to enable cross-script access. Basically the same thing, except myVectorLine becomes FindObjectOfType(scriptName).myVectorLine
Of course the funny part now is that during the course of trying to resolve this, I've actually grown to like the quasi-easing effect, LOL.
And I might still go in for Curvy, especially since they've just published some PM Actions, and, as you said, it's optimized for mobile. The Magic Box plugin looks pretty awesome too . . .
EDIT / NOTE: I'm declaring this "Closed" and not "Solved" since a solution was discovered, but not one that directly addresses the actual issue. I found a way to achieve constant speed, but not with iTween.