as i know the Proof of stake is about staking an amount of specific coin in the network so it can make decisions what's gonna be in the next block
can I develop a code that I could use this amount of coins while it's under Mortgage?
could I develop a blockchain code that you can stake whatever coin you have?
I'm not 100% sure that this is what you need, but maybe you can check for LPOS (Leased PoS), WAVES has that. Maybe you can mix PoS with LPoS and other whatever-PoS got invented meanwhile.
But I've never looked into the code of any PoS, so I don't know...