Are there penny-farthings with an internally geared hub? Has someone built a penny-farthing (high-wheel/ordinal) with a gearbox in the hub? If not, are there obvious reasons one cannot do such a thing?

(I recognize this is silly, but that never stopped people from doing all sorts of other things.)
Yes, there exists at least one which was made/improvised with a Schlumpf Innovations bidirectionally-fixed internal-gear unicycle hub.  If I recall correctly, it had a 36 inch unicycle wheel, giving an effective diameter in high gear of around 54 gear inches, fairly typical for larger penny.
The structure and silhoutte of course were odd, with the relatively small physical wheel it might be termed a &quot;mini-penny&quot;, the actual fork was an old Nimbus Nightfox unicycle frame squeezed to fit the 100mm bearing spacing of the Schlumpf hub vs the 125mm spacing of the Nimbus Unicycle hub it was designed to hold.
Edit: build description from the creators, alas, no pictures.
There also is/was the &quot;bicymple&quot; which has a fairly small difference in wheel size for a &quot;penny&quot; but does have the bigger wheel up front. It freewheels, but one of the models incorporates a freewheeling version of the Schlumpf hub.