The status of WHH in Lost, I think, is, to first order, that WHH turned out to be real. That was a mystery that if we were forced to accept a particular answer, while the show ran us around on this issue, seems to have ended up confirming that Whatever Happened Happened.
However, someone made a blog on the question of what would you do if you were on the island in 1977, on the premise that the author would, in spite of WHH, try to store a note on the island in a way that they would find it... and possibly warn themselves the their coming fate. By WHH, you would never find this note no matter what.
HOWEVER, it's not so simple. For one, I realized I would try too, if not that, something, but I wondered if that was due to the Lost mythos leaving room for that, or if it was just my own predilection. While I will admit part is my prediclection, I do think Lost still left us reason to doubt a literal interpretation of WHH. The reason I say this is comes from the fact that Lost also certainly settled an issue we call "course correction". If Desmond was witnessing futures where Charlie died, then those PARTICULAR futures were avoided. Sure something of the same type happens, but dying on friday IS NOT THE SAME as dying on thursday. Also, Eloise, she said she COULD have stopped a man from dying, but he would just die some other way. This and many other details point to the fact that you can try to change the future, and you can in fact therefore change a limited segment of time... and theoretically you could push off the course correction for an indefinite time.
Thus, in the Lost universe, I would still believe, as Eloise does, of working to affect the outcome, because even if everything meant to happen happens (in some form)... the difference you can make is to the timing, and as we all know, timing is everything.
Cheers.
PS: me == formerly Harcourt