Even using 406.8 for your argument.I think 406.8 is clear, and it would be a violation. This is a field fabricated cord and must comply to NEC.
If the OP buys a UL listed adapter cord and uses the UL listed adapter to the feed the piece of equipment, the NEC is satisfied.
Question is, does the manufactured adapter cord even fall under the NEC? Does the piece of equipment fall under the NEC?