Take out a margin loan and use the money to get a mortgage?
It’s probable banks would accept stocks as collateral (those that provide Lombard loans, at the very least) but they’d probably ask that you use their services for the custody, meaning higher fees, and would only count a percentage of them for their calculations.
Plus, you’d be exposed to stocks drawdowns, meaning the value of your collaterals could drop, potentially forcing you to quickly find another way to finance the potentially missing part of collaterals for the mortgage.