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This is an interesting approach, but unfortunately there are side effects I've found that may not be intended. The demo has the fast walking beetle, so Mario dies immediately. The beetles don't wake up for a very long time after being hit. But there is one side effect that I like, where the world select is not enabled by default (even if you reset).

However, enabling world select is what really enables hard mode after beating the game naturally.

When starting level 1 it checks the world select flag ($07fc), and stores that value in the hard mode flag ($076a). You could alter this instruction to instead store the accumulator (which happens to be 1) in the world select flag, afterward everything will cascade into place with hard mode enabled exactly as if you had naturally beaten the game. But, once you die or soft reset, you can select any world, since world select is now enabled just like when you beat the game. (Game Genie code SENAIXSX)

However, the game timer ($07fa) should be 1 during this check, so you could load that value into the accumulator instead. When it stores that value into the hard mode flag, everything should fall into place without the world select side effect after reset. (Game Genie code XNNATXKN)