Good option but I have a work around for you.
A)Anyone can change their status after signing up. Thus a player can move themselves to the "I will not be attending status"
B)Anyone that signs up as Late can put in a note that says "Locked to Raid X" or something.
My suggestion will actually go back to something I suggested shortly after Chops added the Late Notes option last fall. Notes should apply to the I will not be attending status as well. Thus players can set the I will not be attending option and indicate that they are On Vacation, going on a hot date, or Locked Out/Saved to another instance.
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Six Demon BagJack Burton: Hey, what more can a guy ask for?
Egg Shen: Oh, a six-demon bag!
Jack Burton: Terrific, a six-demon bag. Sensational. What's in it, Egg?
Egg Shen: Wind, fire, all that kind of thing!