It is actually very simple to implement this without making STEEM NFTs. Rather than time locking each satoshi STEEM, we allow an account to set a maturation date on their Steem Power. When the account powers up, bonus VESTS would be allocated as a function of how much STEEM is being powered up and how much time the account has left before the maturation date. The maturation could be increased at will for most bonus VESTS, but never decreased.

For those unware, VESTS are an internal tracker of claims against the Steem Power pool. An account's VESTS do not change over time. As STEEM from inflation is added to the pool, the value of 1 VESTS in STEEM increases so that when you withdraw it later, it is worth more. On the UI, we do this conversion for you and display Steem Power in STEEM, not VESTS, at the current value of VESTS to STEEM.

For users wanting multiple maturation dates, it is fairly simple to create multiple accounts with different maturation dates and delegate all of the Steem Power to one centralized account that is regularly used.