Good guess for me... I thought that up as I was...
typing, thinking they might be a legal issue.
I think I would add another (lens or light) to the left side as well just to balance the look, whether you light it up or not. Great idea in any case!
Thinking a bit more... if you do and you can't use it as a DUAL fog light, why not wire it in as a 3rd back-up light? I'm sure you could always use the illumination and the "imbalance" of the lighting would only be while you were in reverse.
Lastly... Attached is a photo of my derriere and note the solid RED lenses all across. Technically illegal not to run back up lights... but I rather have MORE brake lights driving around here in LA-LA-LAND. I did it more for the LOOK that I LIKE primarily. Having clear, red, and amber colors, plus the license plate pods makes it look awfully BUSY back there, IMHO.
The directionals are earlier model stockers, the back-up lenses have been tinted red using CANDY APPLE RED (translucent) spray paint on the back side. I learned this trick many, many moons ago building model cars. Keep adding layers of paint until it closely matches the other lenses. Usually you can't go too far... but I found that Lacquer Thinner generally will not hurt this plastic and you can remove the paint if you do go too far. This is NOT true of polystyrene models though.
HA!