Kurt,
I recently bought a modern Pletscher rack from Rivendell Bikes. I've mounted on my 2003 Cannondale Comfort/MTB (26" wheels) and I'm astounded by the stiffness and rigidity when compared to old Pletschers.
Of course it is currently out of stock 
but take a look at the pictures to see how you might add to the bracing on a vintage one to improve the "sway" you experience. Part of the stiffness comes from these
mounting brackets which attach to the eyelets at the dropouts.
I remain fascinated by your project and it would be cool to have one of your film/video friends film you in action while filming Bike to School Day in May.
