[TheDong]

As I understand it, the majority of these future's contracts are on Etherium or another crypto market, and pay out in stable coins at best.

That's already too much risk for 10 years out. The chance of etherium having died in the PoS transition, or the chance of any given market-related contract having an exploit that renders it worthless, are both simply too high to actually participate in for a 30 year loan.

Do you have a reference to a place where I could get one of these futures to get a good feel for how much extra it would cost to secure a loan against that volatility?

[quantumBerry]

I think the non-KYC exchanges may use stablecoins as collateral due to the inability to service fiat. But Kraken for instance has BTC-USD perpetual futures [1].

Presumably a large mortgage lender would write their own contracts or collaborate with an exchange for a product using their own mortgage contracts instead of USD on margin as collateral for the loan.

[1].https://futures.kraken.com/trade/futures/PI_XBTUSD