When I said "a lot", I wasn't exaggerating, but neither did I mean that JWs were doing it "end masse". What I saw was a number of such happy birthday wishes between active JWs from a couple different areas of the US, and one JW even reached out to a worldly relative wishing them the same.
As far as it being judicial, I looked it up in the elder's book and apparently it isn't. Under the list of judicial offences, this is all it has to say about holidays:
"Not all holidays directly involve
false religion and require judicial action."
So I guess birthdays are just like Thanksgiving, April Fools Day and some others in that they're discouraged but you can't get disfellowshipped specifically for it. That being said, everyone would be horrified, you wouldn't get any privileges and it might even turn judicial if you flaunt it, thereby falling under the 'brazen conduct' umbrella.