The new FAQ states the following:
Page 162 – Spell Generation
At the end of the second paragraph, add:
“The Lore of Undeath is available to any Wizard.”
(see Warhammer Battle Magic: Lore of Undeath or
Warhammer: Nagash, available in digital format at
www.games-workshop.com)
Now since that's under "Spell Generation" I don't know if that applies to Loremasters. BUT, the fact that it says "available to ANY wizard." To me would mean if they are considered a wizard and can cast spells, they should be able to take Undeath.