Someone actually tried to follow this advice starting in September 2007: https://www.bogleheads.org/forum/viewtopic.php?t=5934 . Needless to say, it did not work out well for them and they almost went bankrupt due to spiraling margin debt.
The difference between a mortgage and margin debt is that mortgages aren't constantly marked to market, and you can continue to own the house even if you're temporarily underwater on the mortgage. Whereas with margin, you can be forced to sell if the value of the assets you've bought underperforms.
The paper that was linked suggests using derivatives rather than directly taking out loans, which are financially convertible. Specifically it says buying deeply ITM call options for several years out strike date.
The difference between a mortgage and margin debt is that mortgages aren't constantly marked to market, and you can continue to own the house even if you're temporarily underwater on the mortgage. Whereas with margin, you can be forced to sell if the value of the assets you've bought underperforms.