Nec absolutely does regulate what you can do with flexible cord, see my edit for the appropriate sections in the code book to reference. Number 11 on the list of appropriate uses in 400.7 is where you will find the exact wording that specifically prohibits this device.

And no it's not legal, not even in the slightest. you cannot wire electrical devices for use outside your own home without an electricians license or working directly under someone with one, full stop. you cannot create devices that intentionally violate the NEC, regardless of if they're for use in your own home or someone else's, full stop.

Edit: since I don't actually expect you to read 400.7, here's the part you should be concerned with

Between an existing receptacle outlet and an inlet, where the inlet provides power to an additional single receptacle outlet. The wiring interconnecting the inlet to the single receptacle outlet shall be a Chapter 3 wiring method. The inlet, receptacle outlet, and Chapter 3 wiring method, including the flexible cord and fittings, shall be a listed assembly specific for this application.