Here's a progress report for those who had an interest in this project.
I've made a mockup of two footboards, using 12mm plywood bolted onto the existing footpeg bases, and with an aluminium sheet lip on the inside to minimise hot airflow to the inside of the pillion's feet. Chris tried this in sitting position without riding, and has suggested some modifications, so we haven't got to the "Road Test" stage yet.
I will therefore make second pair of mockup boards, which will be square at the toe, just on the inside, with a straight heatshield lip, and the whole board angled in just a few more degrees. The last photo roughly shows the intended shape formed in in black masking tape.
Once I've made and fitted these, we'll give them a good run to test the shape, as they are strong enough to support the necessary weight, but don't have any vibration isolation. If this works OK, the final version will be fabricated and bent from aluminium checkerplate for the base and heat shield, with two layers of rubber for the vibration isolation. The bottom layer of rubber will be thick and soft, and the top layer will be thin and harder.
I'll post some more info as the project progresses.