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