Does it violate US wiring code to have non-standard outlets?
There is no listing for non-standard receptacles, so at a basic level, it would require "examination" by the AHJ (Authority Having Jurisdiction) in your area under NEC Article 90.7 to make sure it doesn't violate the basic "suitable for use" rules in Article 110.3. You might find an AHJ who will accept the IEC certifications as adequate, you might not, there is no way to know in advance. Most states have specific rules as to what third party listing agencies they will accept, often deferring to a list maintained by OSHA, referred to as the NRTL (Nationally Recognized Testing Labs) list. But unfortunately, most IEC devices do NOT undergo testing by any third party listing agency, manufacturers can "self certify" to any applicable standards, something that we in North America don't accept. I would venture to say that MOST inspectors here are aware of this and will not accept IEC devices, but again, you can't know without asking THE one that is going to inspect your house.

All of this is predicated on you having it inspected at all. If it's just you living in your house, you can do what you want. But if there is a fire, injury or loss associated with the device, your insurance company may use the unauthorized installation of it to avoid having to pay damages. Then when you go to sell your house some day in the future, you would have to remove the devices or disclose to the buyers that there was unpermitted electrical work done, something that scares a lot of people.

Another aspect is the equipment you intend to plug into this device. First is the 50Hz / 60Hz issue; if there are any motors inside, they may not work correctly. Then there is the fact that 240V here in North America is Line to Line, 230V overseas is Line to Neutral, with the Neutral referenced to Earth (ground) the way we do with 120V. While normally they would have the Neutral and Earth separated, in rare occasions the Neutral of equipment is bonded to earth inside. In that case, it's impossible to connect it here, it would be a dead short to ground.