If you knew you'd want to timelock something before moving forward I don't see much of a coding problem. What would be a real issue though is the UI as spells change, yes, this would essentially force them to load a separate client for the time locked content, but that beats having to rebalance things every other patch, and it beats content dying. Building an additional system that lets you customize your time locked UI outside of the actual content would be at least as big of an effort as the time locking itself.