Well, I've solved it...at least for my usual doubles partner. I ended up buying a sheet of dense foam insulation used in building construction, and measuring and cutting two pieces per footwell to be sandwiched together. These block out the space behind the flexy footplate, giving an extremely solid platform to drive off of, while also assisting in filling some voids that would otherwise fill with water. I added Velcro strips to hold them in place (or remove them easily), and made 'v' cuts with a box cutter blade to allow drainage channels for the venturi drains. Unfortunately, it's good only for someone at this leg length, but I imagine it would be easy enough once the forward angle is cut, to Velcro on additional layers (or remove) to allow for differing leg lengths. Works great.
"Nice? It's the only thing," said the Water Rat, solemnly, as he leaned forward for his stroke. "Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about in boats."
'The Wind in the Willows'~Kenneth Grahame