No baelonorns, as they are not part of the open game material. Like the beholders and mindflayeers, they are solely the IP of WotC. However, I do discuss elves as liches.
There is a discussion of "good" liches, and thee creatures are rare and tragic.
I reference some folklore stories about people who removed their hearts (or spirit, or capacity for death) from themselves and hid these items.
I expressly discuss Koschei the Undying in the book.
Liches as PCs are possible, and something I explore in the book. They are some challenges to playing a lich. For one, liches tend to evil, and so what kind of campaign could you run a lich? Also, they tend to be powerful, so what is the power level of the campaign?