No guarantee past performance will equal future, so not sure how looking at a specific past time period would help. Also the HELOC line available could be reduced or interest rate rise, at worst possible time.
Sure it could work out or even perform better than a conventional approach but you would be increasing risk.
Instead of having a portion of portfolio in bonds, cds, or cash to reduce risk, this would be investing in stocks with borrowed money. It may yield a better result on average over time but with way higher variability and greater chance of worst outcome. Reducing that variability is why people invest in assets other than stocks.
Actually - no, its not riskier. Might seem counterintuitive, but frequently the calculators say that for a long retirement, going to 100% stocks may actually reduce the chance of portfolio failure slightly. The longer the timeframe, the riskier it is to have bonds - because (on average) they yield less than stocks.
Sure, HELOCs were closed or reduced. Really, that's NBD. It will probably take some work, but there were ALWAYS banks/credit unions willing to give out a HEL/HELOC, even in the worst of the times after the housing bubble. Go open another one.
Or sell stuff on Craigslist.
Or get a part time job.
Or do some other side-gig for income.
Or (heresy!) live on a low-interest credit card for a year.
It's one of many potential options to avoid selling stocks when they are 50% down and shouldn't be rejected because of a kneejerk Dave Ramsey reaction of "debt is BAD!" - debt is a tool. Uncontrolled debt is bad. Properly used, debt could prevent that early-years failure of your portfolio.