In Bitcoin if you solve a block the coins that are part of the reward (transaction fee + block subsidy) are unspendable for 120 blocks.  If you try to spend it the client will stop you. If you hack the client then the transaction will be seen as invalid by rest of network until the inputs are at least 120 blocks old.  (technically it is 100 blocks but that is another story).  This is done to prevent orphaned double spends.  If you could spend generated coins (coins produced from thin air in coinbase) right after generation then you could "pawn" that risk off on someone else.

I solve a block #177,777 = 50 BTC.  Sweet.
I buy something from you for 20 BTC.  You ship it.  I pay you the 20 BTC from my block reward.
A different fork of the block chain ends up longer.
All nodes replace "my" block #177,777 with the version solved in the replacing chain.
My block becomes orphaned.
My/now your coins disappear.  They never exited because as far as I never solved the block #177,777.  

Either way the coins are "lost".  Bitcoin limits that lost to the miner by making the coins unspendable.  If a re-org occurs it likely will be within 120 blocks after generation (forks greater than even 6 blocks are extremely rare outside of an attack) thus my coins disapear before I can spend them.

Given this already existings in Bitcoin it can be adapted to form a built in "stake escrow".  Lets say for a proof of stake you wanted the "staked" coins held in escrow for 2016 blocks.

Bitcoin Coinbase:
Input = 0 BTC
Output = Reward + transaction fees (currently ~50 BTC).
Coins can't be spent for 120 blocks.

Proof of work/stake hybrid Coinbase:
Input = stake
Output = Stake + Reward + transaction fees (~50 BTC + stake amount)
Coins can't be spent for x blocks thus forming an automatic and irreversible escrow (x=2016 in this example).