I'm afraid I can only offer a partial answer, the first part is problematic:
Can anyone provide some references as to when this meaning of the word a appeared?
Personally, I can definitely recall the term being used during the controversy around the cover of the US edition of Justine Larbalestier's Liar, which was around May or so of 2009. (Bloomsbury decided the best way to represent the story of an Africa-American girl whose hair is referred to as "nappy" more than once in the book, was to put a picture of a white girl with long straight hair on the cover).
Thinking of political uses of the term in relation to race, I can think of a few just a few years before that using it in the sense of "covering up how bad something looks".
Not a full answer by any means. The Liar case got quite a bit of publicity in some quarters, not least because its author was one of the people protesting which made for a good story as far as reportage went, so it would have been more likely to get above my personal radar than other complaints about the same thing (I do pay attention to issues of representation in art and popular culture, but not as much as many other people do).
It is quite possible that someone out there knows exactly when it came into being, or at least can think of a good example earlier than 2009, but it's also difficult because of the existing meanings of "whitewashing" with political implications makes it harder to trace a given single use.
How widely accepted is this meaning?
It's pretty common in groups who are likely to be talking about the issue. That would mean that people who talk about racial issues would likely be familiar with it, and people with a strong interest in the mechanics of popular culture and entertainment would be familiar with it, and those in the overlap of those two groups would of be very familiar with it.
How [in]formal is this?
I'd say reasonably reasonably informal. I'd suggest putting it in quotes if writing something very formal unless aimed at an audience that would be well-familiar with it. I'd suggest not putting it in quotes if writing something at all informal.
The Daily Mail put quotes around it in their headline. Why?
For one thing, they are mentioning rather than using, in stating what someone else has accused L'Óreal of. You'll notice that quotes are often used after the word "accuse" in headlines, even with established words like "stealing" or "killing".
For another, while the Daily Mail is not quite the same newspaper as it was when it was officially alligned with the British Union of Fascists (it was owned by the leader of the BUF and actively supported Fascism up until Britain went to war with Nazi Germany), it still markets itself primarily at people who hate immigrants, are at least suspicious of gay people, people of religions other than the larger Christian denominations, and convinced that everyone is in a massive conspiracy to live a life of ease and debauchery all while living solely off benefits paid for by white lower-middle-class male tax-payers. For the most part these are people who would only even use the word "racist" as part of the clause "I'm not a racist but...", rather than be familiar with even the more common terms used in terms of analysis of race-relations issues.
