I thought about that, but would actually have to see a body superimposed on a wheel to figure out if it would work - my gut feelings was it would be awkward, but could be wrong. Also, the extremely experienced unicyclist who built recumbent unicycle reports that the reclining position really complicates balance - technically in a penny you aren't trying to do that, but I suspect it would still be quite unnerving.

Another possibility might be some sort of spherical-bearing splined thing, to allow a sprocket on the steering axis at the top of the wheel to be driven by a chain from behind it. Not having ever actually had the privilege of climbing onto a penny farthing, I don't know what sort of range of angular rotation you actually need for steering, but obviously it can't be too extreme.

Where I to suddenly find myself in possession of a backyard fabrication shop, I'd probably go for the Seuss-esque parade triple: rider inside, rider atop, and someone bringing up the rear on the farthing. Possibly with three the captain could just steer and not contribute - so basically a monowheel with a small fork to pass the rim on top, joined by a long swooping penny frame to to a kiddie tandem with a layback seat to fit an adult, and passive seat with steering bars up top for the captain.

And each needs a big honking horn, if not an actual brass instrument.