Every month, your employer transfers Bitcoins into a time-locked address that can't be transferred out before you reach retirement age. You have the key to the address. You can verify the transfer on the blockchain.
Intriguing, but probably won't fly.
All Bitcoin-Hype aside, for easiness' sake, let's assume we're doing this with our own, special coin, let's call it RetirementCoin(RC).
In the beginning, people will need a lot of RC to pay into the pension fund. RC is new, its price is low, i.e., for a given amount of e.g. US Dollars, you'll have to buy a lot of RC and lock them in your address. Exchange rate of RC will go "to the moon", leading to lower amounts of RC required to put into your retirement address.
The years go by.
Finally, when pay-outs start, huge amounts of RC from the first pay-ins will pour into the market, pushing the exchange rate down.
Your second pay-out will be substantially less RC at a substantially lower exchange rate. You'll be practically broke once you retire.
and why don't we just hold bitcoin forever until we retire? RC does not guarantee the exchange rate will always grow in the long run. bitcoin also does not guarantee, therefore I say that there is no guarantee of a good pension in the crypto world. except if we work in certain government or company institutions.