LARP's origins date back to well before D&D was invented. Kids have been having swordfights with sticks and pretending to be pirates for a long time, so there's a long-standing desire to act out your favourite saga, story, film etcOne thing I had failed to mention in my earlier posts is OTL the hobby sprang from full adults, able to print books and found businesses, which meant they had both the higher maths literacy and the incentive to use harder determination numbers as 'spice' to make their systems feel better/more realistic.
If we want to found a whole LARP movement out of something else, it probably has to start off as an offshoot of psychological research or as a provocative arts experiment that 'becomes viral', so to speak, and makes a whole new genre of entertainment as an offshoot. I still have doubts about the feasibility though, as without a shared understanding softer boundaries would be more exploited and bring discord (lowercase 'd') to the fore.
For adults, reenactment societies provide the opportunity to wear armour, see what fighting with realistic weapons and formations etc was like. The Sealed Knot was founded in 1971, and my first D&D group included Sealed Kot members along with figure and board wargamers. A few years later, the local fencing club put on a semi-historical display for local summer fetes. It wouln't have taken much to spark'lets take these outfits (or let's make our own), go out to here and have a forest dungeon adventure for real'. Thinking up a couple of basic magic effects wouldn't be too hard and there's your LARP. from naturally occuring ingredients. This didn't happen OTL but similar situations with fortunate overlaps of interest can't have been that rare.