UCI issues aside, if you're going to go around bolting things on to make it make it more areo, why not do it properly and start with the main problem, which is the position of the rider?
You can do something about the rider’s position without even changing the bicycle or bolting on fairings: the racing crouch.
Failing to do this properly is the thing that surprises me the most about the cyclists I ride with. They have fancy bicycles with aero wheels but they don’t get low enough, even when speeds wind up over 50 km/h. It’s pitiful to watch a strong man push the pedals with unfathomable force and go nowhere because he’s sat up at 45 degrees. The draft behind these guys is enough to asphyxiate you.
Something I notice about Froome, because I do the same thing, is that he gets low every time he pushes the pedals, even if he’s just narrowing the gap from 2 m to 1 m with a three-second effort.
This photo is typical: he’s the tallest man and yet has the lowest back and the lowest head.
Judging by the people I see on the roads, there aren’t many cyclists who wouldn’t benefit from getting their torso down a bit. Perhaps worry about fairings (and aero wheels, etc.)
after picking this low-hanging fruit.