Are there any lenders who would take them as collateral if I needed to access some cash over a multi-year timeframe?
Don't view the lender as doing you a favor. It's you that has to trust the lender not to die or scam with your coin. And on top of that trust, you will need to pay high escrow fees since there is a chance the value of the collateral will go down.
You should give the coin to a local law office, and they can issue guarantees to the local bank for so much money. They can even do it anonymously for you, so the bank isn't aware of your identity. And their fees will be cheaper or comparable to random dudes (like the guy above me) who, based on his feedback, has no problem promoting ponzis.