Would "Mad" Mike Hughes' steam-powered rocket have been certified airworthy by the FAA?
Interesting topic.  Rockets and spacecraft are still highly experimental vehicles and are recognized as such under 14 CFR §1200.  The laws, liabilities, and regulations for private spaceflight are largely uncharted territory at this point, though §1266.102 requires all participants aboard the ISS and NASA launch vehicles to sign a liability waiver prior to flight.  Essentially, in the eyes of the law, you’re on your own in doing this and if your injured or killed, neither you nor your estate can hold any participants in your cosmic ordeal liable.  Given that many flat earthers view NASA as the enemy of their efforts, any laws in regards to spacecraft certification would probably be viewed in a negative light and disregarded.

Using §91.7 here is pretty nebulous as there is no definition or standard here for airworthiness of such a craft.  These guys were testing an experimental vehicle at a remote site far away from other persons or property and flying only a single flight crewmember.  There’s really no law saying you can’t take a risk like that under those conditions.