Heh... well, my stoker isn't legless but she's only 6yo, the tandem is electric, and the rear crank will run faster than my one up front. So that'll be fixed footrests above an armless crank.

Having concluded that this isn't really a thing, i have this morning gone ahead and made my own. My tandem's cranks are some pretty standard early '90s Sugino GT 110s so no worries chopping one up (note the severed arm on the tool box).
All three cranks from the bike look identical, but only the one in question takes three rings, and the other stoker crank is 5mm shorter, so i had to get it right first time.

And finally, after plenty of filing and sanding...



It needs a good polish and i'll be x-drilling the chainrings, but it's turned out fine.