Are internally-geared hubs UCI legal? I am assuming no one would ever use one for a race since durability and reliability matter less in a pro race than weight and efficiency, but I am also assuming that the UCI nevertheless put some stipulation in its rules to address them, just because. If so, what part of their code pertains to this?
Here are the UCI's rules governing bikes (jump way down to 1.3). After a quick perusal, they seem to be silent on the subject of internally geared hubs or multi-gear mechanisms in general, which is surprising considering how detailed their rules on sock length are.
They do, of course, prohibit multiple gears on track bikes.