No... No transparent cube is competition legal, because it allows you to see more pieces than on the original. This violates the modification rule in the regulations.
I guess it would be up to the main judge to examine the cube in question and evaluate if there is any extra benefit with the cube. In short he would decide if the mentioned rule applies. Transparency is not mentioned specifically in the rules (except the part about more visible parts/colours). Transparency does not automatically give this benefit, so an inspection would be required IMHO
If the basis of the rule is that you can see the color of the stickers on faces that you normally wouldn't see then why can't you use stickers that have a "white backing" on such a cube?
It's kind of beyond me as to how someone can use color recognition on all sides of the cube. You typically only need to see three faces to know the orientation of all the cubies
He said "Therefore these specific transparent cubes cannot be used in official WCA competitions.". So there's nothing wrong with transparency; some different transparent cubes could be legal.