Lizardmen army book, page 53: "Mage-lord of Hexoatl: Lord Mazdamundi has the Loremaster special rule for whichever Lore of Magic he chooses to select spells from." Okay, so if Maz takes High Magic, casts two spells in turn one and swaps them out with Contemplations for a roll on the Life and Light charts. He chose to select spells from those lores. Does he have lore mastery of Light and Life now? I cannot think of any reason to say no, but it feels wrong. Assuming he did get Lore Mastery of Light and Life, I would say that he cannot recast whatever High Magic spells he swapped out. After all a Lore Master is still a Lore Master even if he loses spells to a miscast or lore of Tzeentch attack but the spell is gone. End Times magic spelled out in Khaine is different, since it specifically states that casters have access to every spell in their lores and Lore Mastery is redefined to mean rerolls not extra spells, so you can't take ALL the BRB lores without losing all eight High Magic spells, but it's still potent. Thoughts?
I can't see any RAW reasons not to take it... but I can definitely see RAI as a reason for not taking it.
You're right Scalenex, Mazdamundi can have all the loremaster rule for all lore. Games Worshop made it for that, that's why he cost so much (780pts). It's like if you put a Slann with Wandering Deliberations and he's at 3" from a wizard tower. He will be a loremaster of the 8 lores from the BRB.