Should be done piecemeal. Granted its the type of game that isn't really lore dependent. You can play the game without even considering the lore, more or less just some extra stuff for people really into the game.
Could be really useful if Riot implements more in-game lore related stuff like the Kha'Zix/Rango icon, Cait/Vi, Lissandra/Seju/Ashe, etc. Or creating more lore related skins.