Could be done using multisig and without timelock: 1) Generate priv keys using entropy that on avg requires X months of work (very similar to block difficulty). 2) Derive pub keys. 3) Destroy priv key. 4) To spend you now need to re-generate priv key(s for multi) which will on avg take X months. Bonus feature: never need to re-new spends etc., because countdown to spend starts when you start grinding.