Fixed-income products are generally priced in yield, even though the actual agreements mention dollars. The futures for treasury bonds are literally denominated in yield points, but most (fixed-income) things it's more "people denominate in USD and think in yield".
That is, everyone trading bonds/certain commercial real estate/mortgages/preferred stocks/etc. converts them to a % return, with different yields falling in different risk categories. So you might get 3% on a relatively safe commercial bond, 10% lending to Turkistan, 4% on a property in the middle of a desirable city, 13% if it's filled with asbestos and shut down by the city.
It would be possible to price a mortgage in any continuous ratio between USD/Bitcoin, though I suspect the imputed interest rates for an already-low mortgage rate would make it undesirable. Someone would just have to punch current prices into a spreadsheet to see how it affects the yield.
I would expect Bitcoin specifically to have a high interest rate holding a long position(since Bitcoin people want high leverage), and the short position to have an implied positive yield. So banks should want to price in Bitcoin since they like leveraged low-risk low profit. But I haven't looked at a price chart for that assumption.