I have the book in hand and am very pleased to inform our community that the Undead Legion rules and the Lore of Undeath are 100% legit (as much as any other Army Book or Lore) for use in ALL games of Warhammer.
You do not, in any way, need persmission to play with the new army. At all.
They make it clear multiple times in just the first 4 pages alone that these are rules designed to be used "in any other games of Warhammer you would like" to use them in.
Of course, as with all things, your opponent can refuse to play you, or your local Tournament Organizer can run in fear and refuse to allow you to play by the rules, but if this happens, it would be the exact same as saying "The Empire book is not allowed in this event" or "Man, your Warp Lightning Cannon is too hard for me to beat, so I don't want to play against your Skaven army."
There is no grey area. GW have actually spelled it out really damn clearly.
Thank Sky Cake.